I believe that pre-war Snyder Mfg. may have used the bottom bracket for actual serial numbers (versus just place and date codes).
Post-war, I believe serial numbers started with an "A" in 1945, located ~6" below the seat clamp; not sure if they skipped either or both I and J.
I was just looking at a 35EH K-4xx,xxx serial number, and although for earlier Snyder bikes, I have wondered if ABCD was for Little Falls NY, and WXYZ for the acquired Indiana plant. I have not read it on any site before, but I am starting to conclude that the "SN" marks may indicate original SNyder New york, and "EH" marks may indicate the former EXcelsior plant in Michigan City IN (X = H in Mexican).