Thanks Stu. Yes, I think the engine in question for me is a M25XPA.
I looked at that link to the C310 thread in your post above. That's where I saw that the -908 does indeed exist....which is the part number the marina mech told me - and this is a heckuva coincidence - and is the mech that is at the marina where KZW keeps his C310. Small world...
FYI for future search; From the Toad website:
M25XPA Model Engine
Produced from: March, 1993 to August, 1996
About the M25XPA series: This engine uses the same parts as the M25XP but is wired like the M25XPB.
So, it looks to me that whatever physical accessory fits an M25XP, it should also directly fit the M25XPA. Of course, to be sure, a call to Toad to confirm would be prudent.