"Reproductions" don't always follow the originals faithfully. For example, you'll run across far more Parker Repros with ST than DT--but it's no contest that more of the originals had the appropriate number for a doublegun: 2.
I've owned 3 Model 21's: 2 16's and a 20. All 3 of them were DT, and only one had a beavertail. Certainly on later guns, BT's and VR's became more common. And I seem to recall that double triggers eventually disappeared on 21's, although I expect you could've ordered them on a custom shop gun. But among the early guns there was no shortage of splinters and double triggers. Because the 21 came along much later than the other American classics (and lasted longer), it makes sense that you see fewer guns with the standard features of those that first appeared half a century earlier--when it was rare to find anything other than DT and a splinter FE.
