This site presents one page illustrating the components of the Lucas OPUS distributor and another page showing the components of the Lucas CEI distributor.  Rather than cluttering up both of those pages noting the differences between them, this page has been created -- reusing the same photographs -- to document many of the differences.

The OPUS ignition system is, by all accounts, an atrocious POS and should be replaced with a better system at the earliest convenience.  There are two popular ways to do this.  Perhaps the most popular is to replace the ignition system with one of the popular low-cost aftermarket systems such as the Crane system.  You can review such a Crane installation in an OPUS distributor at Bernard Embden's site.

The other popular upgrade is to retrofit the later Lucas CEI ignition system to the earlier cars; in fact, there is a kit available that not only retrofits the CEI to earlier cars but does so while maintaining the underhood appearance of the OPUS system for concours purposes.  Since some owners may have found their way to this page specifically to investigate the idea of retrofitting a CEI system to an OPUS car, there are several pertinent comments included.

There are differences in rotors -- but the primary difference there isn't between OPUS and CEI, it's between carb, D Jetronic, and Digital P fuel systems.  The D Jetronic EFI system has a trigger board inside the distributor, and the rotor has a magnet in the tail to operate the trigger board.  This rotor looks like this:

If you hold a paper clip up to the underside of the tail end of this rotor, the magnet will make itself apparent.

Neither the earlier carb V12's nor the later Digital P cars required such a magnet in the rotor.  I don't presently have a suitable pic of the Digital P rotor, but it's basically triangular in shape with a broad flat tail.

There is something called a "green stripe rotor", but I have no idea what that is.

The rotor carrier for the OPUS distributor also holds the plastic disc, and it looks like this:

The rotor carrier for the CEI distributor also holds the iron star wheel, and looks like this:

For those seeking to retrofit the CEI to an OPUS distributor, reportedly the iron star wheel can be installed on the OPUS rotor carrier by providing a simple spacer underneath it.  You probably don't want to be trying to fit the CEI rotor carrier into the OPUS distributor because it'd be a nightmare trying to work out the differences in the centrifugal advance mechanism.

Speaking of the plastic wheel, here's what it looks like:

This is the bottom.  It looks like it's made of cardboard, but presumably that's just because it's 30 years old; presumably it's plastic or phenolic or some such with 12 ferrite inserts around the perimeter.

I don't presently have a pic of the iron star wheel used on the CEI, sorry.

The rotor carrier in the OPUS distributor is retained by a simple screw in the top.  This is the shaft that holds the carrier:

Of course, Jaguar couldn't leave well enough alone, so with the CEI the screw was replaced with a plastic clip.  The end of the shaft shows the revised configuration to use that clip:

While we're at it, note the differences in the centrifugal advance weights between the CEI versions shown above and these OPUS versions:

They really bear no relation at all.  On the OPUS, the posts that hold the centrifugal weights and the posts that hold the springs are different posts.  On the CEI, they use the same posts for both tasks.  The centrifugal advance mechanism is a totally different design.  Even the springs themselves are different; here are the springs from the CEI:

Don't just sit there, count coils!  Yes, the CEI springs have fewer coils on both springs -- which probably makes them easier to stretch out of shape, a lovely factor to have in combination with the lousy rotor carrier clip!

The vacuum retard module on this particular OPUS distributor (1973 XJ12) attaches to the phenolic swivel plate via a simple pin protruding from the swivel plate itself:

The vacuum advance module on the CEI connects to the swivel plate exactly the same way -- except everything is different.  Again I fail to have a suitable photo, but the rod on the vacuum module is different and the swivel plate itself is different.

The OPUS housing with the swivel plate and vacuum module removed:

This is from a carb car; a later OPUS would have mounting bosses for the D Jetronic trigger board.

The CEI distributor, besides having mounting bosses for an "anti-flash shield" (which may exist solely to give purpose to existing mounting bosses that were originally for the D Jetronic trigger board), also lacks the center boss for mounting the phenolic swivel plate.  The CEI has a steel swivel plate assembly with rollers in it, and it mounts to this housing with screws.

Down at the bottom of the distributor, here's what the drive gear looks like from a 1973 XJ12:

Here's what the drive gear looks like from another distributor, later but still OPUS:

I presently don't know for sure because I didn't check when I had one in hand, but I believe the later gear is made of steel.  I do know for a fact that the 1973 XJ12 gear is made of solid brass.  I suspect these two gears are interchangeable.

There are all sorts of ways to retrofit the CEI to the earlier car.  Perhaps the simplest is to retrofit the entire distributor, but one would need to take possible differences in the advance curves into account.  According to Roger Bywater, the centrifugal advance curves on all the distributors prior to the Marelli ignition system -- regardless of model year or market -- were so close to one another as to be within tolerances.  The vacuum advance is a different matter, though, with some having retard while others having advance and the H.E. having more vacuum advance than the pre-H.E.  So, while simply installing the CEI distributor to an earlier car may provide a suitable centrifugal advance curve, you will probably need to figure out how to retain the original vacuum advance scheme.

The safer method would be to retain the original distributor with its advance mechanisms while merely retrofitting the pickup and iron star wheel from the CEI.  It will still probably prove best to retrofit the entire housing so as to get the later style steel swivel plate to hold the CEI pickup properly, and the holes to mount that swivel plate.  That still means you'll need to figure out how to adapt the earlier vacuum module to this housing and get it to attach to the swivel plate and move it properly.