Mate thats a question.... here is part of a statement from a respected Risti...
You start on PreVs which has a different bezel construction to the Pre-V 5218-209, Pre-V 5218-210 and pre-A OP6500 these have the same case & crownguard.
The case and crownguard are a little different for the OP-6502 reference (50-60% of pre-A, A-series, and approx. 50% of B-series), although only a trained eye can notice the differences.
Halfway the B-series, they split-up the case references: OP6518, OP6519, OP6520 and OP6521 are a little different from OP6502 cases, but they continued using the same OP6502 crownguard.
...Then I believe cases changed again around F-G but of course these are manuals the autos have a larger base to the bezel to allow for the movement...
Here is a PreV and Auto (Courtesy of Risti Reference)
Having got some great info from Mike, He explained that in fact the JF case bezel is actually closer to the PreV than the contemporary that its suppose to replicate, I had always assumed these to be bigger from pics I have seen..
From what I have read it all comes down to which series you want there is no hard and fast rule so do you have a particular case you are trying to replicate??