"The "B" model Mariner
"The "B" model Mariners were built by Yamaha for Merc.
They were carbon copies of Yamaha's own model 40A which only saw production from 1977-79. Yamaha did produce their own 40B model but it was only available as a jet drive (during the same 3 model year run as the A's).
Yours is a 1978 model so it was built during the Yamaha model run. They continued to produce the A and B model Mariners for Mercury until 1981 when they pretty much ran out of parts.
They were replaced with the more advanced "C" model which saw production all the way until the early 90's.
A minimal assortment of parts is available from a Merc dealer, however, these models are pretty much orphaned - they do not resemble the C models at all and they are very difficult to retrofit.
Because of their limited production run I consider them a somewhat "disposable" motor. Decent motor if they are running but if anything major goes it's best to scrap it and buy something a little more modern...
Sorry the news isn't a little better."