Mariner 40hp year

can anyone tell me the year of this Mariner 40hp 676-L400566

Old

That is an 1977 "B" model Mariner built entirely by Yamaha for Mercury.

The "B" models had a short (market) life and were replaced by the very popular "C" models - unfortunately, the parts do not interchange which leaves the A and B models somewhat "orphaned".

Yamaha also produced this model with their own decals, but it only lasted 3 model years (1977-79), so likewise, their parts supply is now over 30 years old as well.

If it's running well keep your fingers crossed (and maintain it the best you can) because once she dies or needs a major refit you will probably find that replacing it completely is the more cost effective option...