I bought some Oneway Chucks that came with quite a few jaw sets, adapters, and this thing. I can see it fits the outboard spindle on my lathe (after I took the handle off) but I have no idea what it does. Sorry for the stupid question.

It's a Oneway Mounting Adapter designed for the Oneway Stronghold chuck.

It allows the Stronghold chuck to be mounted on a Powermatic 3520 B or C lathe.

Why would you mount a chuck on the outboard side of the spindle?

Fred, outboard turning is for pieces too large in diameter to turn between centers.

This video shows how the outboard turning would be done. Your adapter is threaded on, as you noted, then the chuck is added to the adapter. chuck would hold the work piece.

Lew, I'll continue to show my ignorance here. This thing doesn't have any threads on it. it's a smoothbore and fits on the other side of the headstock (I though that was the outboard side?) Once I remove the round handle. It does have the taper lock screw holes to mount to the chucks, which only confuses me more. Apologies for being an idiot.

OK guys, I have the answer. It doesn't really fit onto the chuck (as far as I can tell) but is made for a vacuum chuck setup. Thanks to all, but I found this statement on the Highland Hardware site:

Note: The Outboard Adapters at the bottom of the list are used for mounting the Oneway Rotary Air Fitting to the outboard of the lathe headstock when vacuum chucking.

BTW, I'm glad i got this since I've been thinking about a vacuum chuck. I already have a pump for my vacuum bag press.

Edited February 18Feb 18 by Fred W. Hargis Jr

Well, Fred, we both learned something today! Thank you for educating me!!

Part of what befuddled me was the Stronghold name. I had assumed it was something related to that chuck model, but it isn't I guess those screw holes that match the taper lock setup are for holding that rotary connector to the adapter.

Me too. I'm not familiar with One Way products.