I found this. I'm sure it's nothing new to many of you, but it was news to me. Any info?

Oops, hit "post" by mistake. Anyhow, it's a Suntour and it's engraved B.M.X.

It's for the Hole shot two speed system...

That's the freewheel for the old Suntour HoleShot two-speed setup. Now all you need is the shifter and derailleur.

These were cool in that they worked with the two cog freewheel setup as opposed to the Browning two-speed sytem of the same era that utilized two chainrings.

Two ways of two companies trying to skin the same cat, as it were.

The way the hole shot shifter worked was you held the shifter (looks like a brake lever) out the gate, then pedal 3 - 5 ish, simply let go and it shifted into high gear. I got a brand spankin' new setup still in the packaging!!

I set my nephew up on the ACS version a couple of years ago and it worked great. I used a mountain bike shifter/brake combo with a 105 derailer. The only time it failed was when someones shirt hooked the shifter and it shifted into neutral!! (followed by a face plant, a lot like breaking a chain)

You could do the same thing with the one you have, I found the shifter had to be a 6 speed shifter (only!) and use a derailer chain.