The sideway stamped P is usually Professionals, and the C usually Corsa's. I'll go out on a limb and guess its transitional, and probably late 85 or 86.
Odd bottom bracket -- don't think I've seen a Merckx of the era without some built in cable routing on the bb -- be it grooves, circular stops, or over the bb tabs. Production was still fairly small, so there are tons of little variations in bottom bracket types, seat stay finishing, fork crown, etc during the mid 80's. Even still...any signs of over-the-bottom-bracket cable routing? Could also just be a leftover bottom bracket from before their switch to routing under.