Whether they used the frames in order or not, I doubt very seriously if you will ever find a 21 with a serial number in the 16XXX range with the old semi pistol grip as they were called. Especially since they quit making the stock in 1933 about the time #8,000 frame was produced and frame 16,500 wasn't made until around 1940. So you have seven years of new buttstocks being made before his frame was ever made. So as I said, his gun is too new to have the old semi pistol grip (or canoe paddle as you call it) on it.
