Hello - and many thanks for the reply. Yes, it is a wraparound cowl (blue band). And no - it was in a tank rather than on muffs.
I've solved one of the mysteries - the seal in indeed still in place. It was just camouflaged with grease and lack of daylight. Phew. I thought I had lost the plot (not that this concludes otherwise).
I have been looking on google, and the best thing I've found is this:
More fun.... Drive Shaft Spline Seal. How's this look?
Do you think the 2 parts could be the equivalent of these 2? One is plastic (same as the metal stopper) and one is rubber (same as the seal)...?
Does anyone know if these are standard parts on an old mercury? If so, should I thread them onto the driveshaft and then insert? Clearly trying to position them in the powerhead would be nearly impossible, especially with my sausage fingers.