There's 3 different size (diameter) choke tubes for the Lyman PowerPac,,small,,med,,and large.
Each with a different recess at the front (or lack of it) and a different shaped 'seat' behind the threads.
IIRC the med and large were both avail to fit 12ga. But it was generally large/12ga, med/16 and small/20ga.
Tubes are marked on the end with the choke constriction in general terms 'short', 'medium', 'long range'.
Most have the gauge marked on the end also,,but not always!
By the looks of the choke body mounted on the M20 in question,it appears to be the Large one.
A slight recess at the front edge and a shoulder behind the threads IDs it as the Large one.
(The 'Medium' size choke device does not have a recess in front of the threads,,it does have a shoulder behind them but it is a tapered shoulder.)
Thread OD on the large tubes is something like 1.15"/.20".
I see them at shows quite often but not at giveaway prices. They're usually priced at around $30 to $40 /per tube depending on condition.
They came in a box w/a small thin flat wrench to install and remove them.
A couple of tubes could easily set you back more than you have in the gun!
If you do cut it off,,save it and the short section of bbl it's attached to.Put it on Fleabay,,it'll sell.