One or two minor things were retro-fitted to the older Mk1 cars but the major changes were all introduced to production at around the same time; as with any model there is a date when the revisions are introduced all together as it is easier to do it this way rather than drip feeding them one at a time.

So cars with all the changes on from the factory could well be considered a different phase of production.

There are changes other than the visible one though, for example the dampers were revised, from memory with larger diameter rods to make them more robust? Final cars were on Rev. D dampers from what I remember.

You can track the changes through the parts catalogue if you want as this shows the different revisions on the parts and the dates they were introduced.

I like to think I have some familiarity with what was done (even if it was a few years back) having had access to a few of the Mk1 cars, including driving #0001 when it was still fresh and having bought one of the last couple of cars built with everything at the final revisions.