To be more clear.
Once you unbolt the original stock, look in the middle of the flat on the bottom of the trigger group.
There will be a stud sticking up that's threaded.
One of the stock bolts screws into that stud.
Some of these are pinned in and can be easily removed by driving out a cross pin, some are apparently machined as one piece with the trigger group.
If you simply remove the stud, you may have problems with the take down frame latch catching in the hole that's left.
The best way of dealing with either type is to cut or machine it off, making it flush with the flat on the trigger group.
You can machine it off, or cut off with a saw and grind and file it flush.
Then all you have to do is drill and tap the two holes and bolt the adapter plate on with some Loctite on the bolts.
Buy a 1921/28 stock with the attaching "iron" and you're in business.