Rich is right, if the boats that were sold to a Mercury/Mercruiser dealer, the dealer would install the power system, but Glastron did have a contract with Volvo, for some boats, as well omc, I don't know any more than that.
One thing that is different now, than then, is that the boats were Sold to dealers back then, not placed at a dealership on a floor plan the way they were powered, or had a contract to do, quite different than the way they are placed today.