Here you go John, a stunning more modern cased gun showing the original tin of Rangoon with others if you read the description, as stated it is a preserving oil, but is often in sets of both rifles and shotguns as you mentioned when I did an image search. I can imagine anyone heading abroad or buying from the Army and Navy stores back in the day would have some for tropical or cold climes on hand, and a chap to apply it

http://www.hallowellco.com/holland%20O-U%2020ga.htm

ATB, ED