The early, prewar guns, have the same stock as the rifles, but the forearms are larger for the shotguns. Yes, the etched receivers like yours were blued. I have not seen any later guns made after the move from Utica after WWII, that were etched. Could you send a picture of your forearm please?
When I was writing my first edition to my book on the M219/220's, I had a list of all the markings I had observed and I asked Mr. Callahan, the Savage Historian, if he would interpret them for me for a fee. He answered in one word, "NO!" So we may never know what the "+" mark and many others are. I recently found one with a German "Maltese" type cross on it.