Further exploration on the 1917/1920
 

First, I incorrectly mentioned that my bbl did not have the calibre marking, which under a glass, it does. The three in the bbl SN does have a flat top. The same digit on the receiver has a rounded top.

The bbl proof is the usual eagle, standing instead of lying on it's side. The other proofs, which I had assumed were "the usual", I cannot find in either the Jones book or Kenyon.

The attached drawing is pretty close. The LH one is larger and appears to be a sail topped by a three part acorn, and a line suggesting a ship's hull underneath the sail.

The other two are smaller and appear to have the three part acorn top with what appears to be a "9" with a tiny seraph on the foot.

I'll post this and see what surfaces after supper. Thanks to all for their comments and help.

Since it is a 1917 DWM, the acceptance stamps are almost certainly crown/T, crown/S, crown/S.