Help! Identify these chokes

[VirtualMarc]

Can anybody identify these 12G chokes? Threads are 20tpi. Can't find anything via Google that uses that thread. Wrench slots are narrow. These are 2.75" long.
Only writing on them is "SK-181 .735", "I.C.", "FULL".

Thanks for your time. Happy New Year.

 

[ss1980]

Home made unknown... If you want to know the constriction though, you can take the gauge measuring tool and drop it in there and see what it comes to.