Older DCS keycaps were made by Comptec and not SP. I believe SP bought Comptec's tooling or something like that, which is how they ended up being the makers of SA and DCS.
That definitely looks like Comptec DCS to me. Although I doubt it's doubleshot PBT as you say in your post, it's almost certainly doubleshot ABS.
That's a really nice find, enjoy the keyboard.