My guess is this bottom plate started out as a 3rd model conversion plate to convert M1917's to A1's or to M1919A4's. These bottom plates had shallow side plate grooves to compensate for the milled off portion of the earlier guns that had peened edges and was excess to requirements and had the side plate grooves deepened to -2 requirements for new made side plates.Very cool bottom plate. Being marked this way really narrows when it was likely produced. The rolled out mark is for the bottom plate drawing dated 3-10-39, D35392 Revision 2, dated 3-19-40, D35392-2. The domed rear rivet on the top plate and flush front top plate rivet are consistent with a gun converted around 1940.
So this plate was made as the earlier style, then modified to the newer drawing spec before installation. I need to look back at your sideplate to see who converted the gun to 17a1 but it probably would have been within a couple of months following 3-19-40. There were a number of arsenals converting 1917s to A1 but RIA produced all the bottom plates used in the conversions
Can you make out the inspectors stamp? Also can you look on the sideplate around the engraving for any ordnance stamps and any RIA, AAB, AAC or any other capital 2 or three letter stamps? That will tell us who converted this gun to 17a1