Myles, sounds like you came across a nice "elsie" years ago. The Record books state there were a few, very few 20 ga. Regular Frame guns made but on one yet has seen one. All so far have been on the FW frame. Know one knows what the R stands for after the serial number, but it seems to be found only on guns with the Curtis for-end release, push button.
As to the 2 3/4" chambers, most likely they were lengthened from 2 1/2", as the 2 3/4" chambers came out in the early 1930's. This is not to say they could not have been ordered as so, but if they were done by the factory, they should have had a oval stamp stating Chambers 2 3/4 Inches on the right barrel flats water table.
Is your Hunter One Trigger position in the rear like on a two trigger gun, left barrel, this could be the reason for the spacer to increase the length of pull.
If you ever come up to the Cabela's in Hamburg, and would like to go sporting clays or skeet shooting, I only live a few miles away. You can email me and let me know. jdw110@live.com