By '3 pin' do you mean two scale pins and an exposed rocker rivet? If so it was made before 1978. I think the rocker rivet was hidden after '78. The knife was introduced in '75. If it really has three pins, then I have no idea.
I can't help with the engraved number. It's possible that it was a gift to someone and that it was engraved by someone other than Buck, the clock number of a retiree or something. It could have been a promotional item, or a special run, although a special run of 150+ knives would have been unusual for knives of that era.
The early knives had maroon Micarta scales, there were a few that were so dark they may have been black, or just a variant of the Maroon material.
Now, the usual disclaimer: There are no absolutes when it comes to early Buck knives. Cutlers, in general, never throw anything away, and Buck was no exception. Prior to their move to Idaho you'd hear a story about somebody discovering a bin of old parts and assembling them into knives. A lot of that kind of stuff got sold to the Buck Collector's Club or to special customers. That could also account for the serial number. It wouldn't have been a club knife as those started in '89 with a 531 Sawby lock.
Never say never when it comes to early Buck knives.