The "bar" proofmarks didn't come into use until 1985. The old "tons" marks were in use from 54-85, and the individual who submitted the gun could request either tons or bars marks during a transitional period, 85-89. After that, bars only until 2005. After that, 850 bar was replaced by "STD". Magnum/superior proof became "SUP".
I've seen 850 bar guns with both 65mm and 70mm chambers. I own a pre-WWII Jeffery 12 that was reproofed at 850 bar with 70mm chambers. I'm guessing that chamber lengthening was the reason behind the reproof. If reproofed in the "bars" era, there should be a bore diameter measurement in millimeters.
