I don't know what the beginnings of Master Volume as an option were, but just for trivia's sake, #73-27 didn't come with one. Someone, (I'm guessing McVie) had it added pretty early on, and it appears again on his '76 Custom Series I with the stainless steel fingerboard, as well as the '76 graphite-necked small-body he had. That Penguin & Porpoise bass with the crazy electronics has what must be a Master Volume too. No idea when that one came through the shop. Total mystery.
I bet it's one of those little extra things that they would always do for you, and sometimes it was noted in a file, and sometimes not. I would definitely consider it an upgrade as long as it's done properly.
If we knew the serial number, it'd be fun to look this one up Keith. I bet you're right. It may have been back home for service and gotten some of these newer parts.