There are conversion adaptor plates available that allow installing a detachable 1921/28 stock.
To install you have to remove the large round screw bushing from the bottom of the trigger unit, drill and tap two holes on the flat, then bolt the adaptor plate to the trigger unit.
That adaptor plate forms rails for the detachable stock to slide onto and lock.
The bad part is the adaptor plates are hard to find and expensive these days.
Tommygunner.com sells the adaptor plate, cost $50.00: