Red: Engine Investigation

Had the day off and decided to audit all the jugs and cylinder heads, starting by pulling the "Mystery" one from Astrid.  Simply can't remember its origin.  I know that I swapped whatever was on Viola/Ivy for the Astrid top end back when we were getting her running, but was that the original Viola top?  I have another cylinder head marked "Viola", but maybe that came as a spare part?

Regardless, I pulled that jug and head from Astrid and brought that up for carpet work along with the "Viola" one.

Stated by inspecting the valves, which I did on the Viola one (everything was within wear limit), but found the Mystery rocker cover screws were frozen and stripped.  Again, not sure why that never got addressed.

Tried to mate the Viola head on the Mystery jug and came to the realization that they are two completely different models.

The Mystery jug has a 68mm surface with no groove in it.  While the Viola has a 65mm cylinder surface.

Are these really 2 different S90 jugs?  I need to lookup the Yahoo discussions again.  I think the Viola 65mm is definitely stock S90 so if anything is different, it's the Mystery one.