In all likelihood the gunsmith who measured the chambers probably used Brownells 10 gauge chamber gauge. It has two lines, one for 3 1/2" and the other for 2 7/8". I will guess that your chambers are 2 7/8" using that tool. I don't know what the 65 stamping means and I don't see any proof marks.
Since this seems to be creating doubt and maybe some anxiety. Instead of wondering what RST shells measure I would focus on the gun. Send the barrels out and have them properly measured, chamber length, bore diameter, chamber length, choke taper length and constriction. I believe it's the only way you will find what you want to know. There was a guy at Hausmann's shoot with those bore gauges, he fixed a dented muzzle on a friends gun while he watched. Maybe somebody else here knows who it was and his contact info. Another idea would be to call the Hosford company who makes those gauges and see what they would charge to measure up the barrels if you send them to them.
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