Could be I'm misremembering, but I seem to recall that the ST became "standard" on the Model 21 mid to late 40's, which was long before the gun became a custom shop only product. (Also true of automatic ejectors?) So while you could still order a DT 21, you don't see a lot of them other than on early guns and a relative few later special orders.

Personally, I like the appearance of the bosses on a 21--as I also do on a Fox. Helps to dress up the boxlock receiver, which can otherwise be pretty plain.