Yep, Set-O-Matics were the norm by then

Looks like the unfortunate addition of some other brand of holder...

That's what I thought, thanks!

That might be a mount from a Ludwig Standard set but Slingerland did have a similar mounting plate at one time. This drum seems a bit late for that set up but it's hard to say for sure.

Thanks, this drum looks fairly late, I can't read the badge number, but based on how it looks, I'm thinking mid '80's. But still, it's in beautiful shape and for a BDP guy (I have two other bdp kits) this is a steal at 100 bucks!

The earlier Slingerland mount I was thinking of had a rectangular plate rather than a diamond shaped one so I bet that is one from a Ludwig Standard. A set-o-matic tom bracket should fit those same holes and could work as a base plate for a SOM arm. It wouldn't be quite "correct" but would be closer than what's there now. The regular SOM base wouldn't work because the holes aren't lined up right for it but the tom plates do go on positioned as a diamond shape and the tom arms and the upright tube are the same size.

For $100 I'd jump on it then figure out a solution later.

Nice drum. For $100 I'd grab it too. Does it have re-rings? That can help date it as mid 70's I think it was the 5 ply shells came into play. You can always look for a Set-O-Matic that is used in a stand. Those have O shaped tubes vs the D shape used in the bass mounts. They fit in the standard size cymbal stands and should fit fine if you want to use Slingerland hardware. This way you can leave the mount on there as is. I use this setup with my Radio Kings.

Makes you wonder what was there first?

Maybe nothing? Hard to see in the pics if anything else was there. Might have been part of a double bass kit? Or a consolette if there's any signs of plugged holes on the side.