What year did chambers switch to 2 3/4"?
 

In the respective Parker Gauges?

You would have to check TPS.

However, I can tell you that I have 1939 12 and 16ga's which are marked " Use 2 3/4" shells". The chambers are 2 5/8".

Tom, You would think this would be an easy question to answer. After searching my collection of Parker catalogs the chambers are listed as 2 3/4" in Remington era catalogs. But in searching the Parker Story pages 164/168 they still specify 2 5/8" chambers for 2 3/4" shells. So I think the answer is the chambers were always 2 5/8" but Remington started marking the barrel lug "For 2 3/4" Shells"

Even though the Western Cartridge Co.'s Super-X 20-gauge 1 ounce load came out in 1922, I have a 1930, VH-Grade, 0-frame, 20-gauge, 235178, that still has the 2 3/8 inch chambers intended for the 2 1/2 inch 20-gauge shells. Anyone have a later 20-gauge chambered for the 2 1/2 inch shell.

Remington introduced their John M. Browning designed Model 17 pump gun chambered for the 2 3/4 inch 20-gauge shell in 1921, and Winchester began making their Model 12 20-gauges for 2 3/4 inch shells in 1926. Obviously some Parker shooters were sticking to the lighter 2 1/2 inch shells.