It is a 1946 gun, if that helps---letter simply says it has #5 engraving---nothing said about custom shop.
I have a 1933 era Tournament Skeet that was rebarrelled by Winchester in the 50's with set of 30" Mod and Full barrels---barrels were marked as Winchester but the wording was slightly different on the two sets of barrels---Winchester even put the original serial number on the new set of barrels.
I'm convinced the barrels on my #5 engraved gun are original---chokes and barrel length match the factory letter.
My guess is that Winchester forgot to put on their traditional stampings.
Bill