Nobody ever talks about 5R cases, Differences between 5R and 7R ? Options

[michel richard]

Whenever a discussion comes around as to which /6 engine case to use for a high peformance engine, the talk always revolves around the early aluminum case, the early mag cases or the 7R cases.
My 2.2 E is built on a 5R case, which is the case that was used for most 2.4 engines, including most 2.4S engines, I believe.
I've never done a detailed side by side comparison of 5R and 7R cases, but I could'nt see differences between a 5R in my hands and pictures of 7R cases. I could see a number of differences between 3R (early mag) and 5R, however.
And I did have to get creative to fit the 2.2E distributor to the 2.4 case (the hole was too big, and the locking mechanism different).
So what is it? Do people forget the 5R because it's close to the 7R, because discusssions have just not gotten around to it ? because it's a little rarer ? or another reason ?
I'm asking because I'm beginning to contemplate a short stroke 2.5 project, and I would really like to keep my case.
The advice is often: go to a 7R, it'll end up cheaper etc . . . But I had to go through 4 cases before finding the one I have - finding good cases is harder when you're not in California, so that starting with a known quantity is a definite plus.
So, the question really is: can anybody help me figure out what the differences are between 5R and 7R cases ?

TIA
Michel Richard