Model 21 Sling?|Winchester Shotguns|Forum|Winchester Collector

A guest at the club this weekend was shooting a M21 12ga that was equipped with what appeared to be a period sling – front swivel and rings were mounted on barrel. Watched from afar so didn’t take much more notice until I later found out he wants to sell the gun. Apparently it dates to 1933. Other than that I have zero info on grade, condition, etc. I did hear he wants $5K or so. I did some quick searches and can’t find if swivels and slings were ever an option? Can anyone advise before I pursue further info on the gun?? Thanks!!