From what I remember the beavertail lugs came out in about 1940 so it's after 1939, a look at the badge and whether it has little adjustment wheels on the snare but will help more to give it a rough date.
The finish is a bit ratty but that bothers some more than others, make sure you get a good look at the interior, RKs are notorious for splitting of the reinforcement rings and that makes a difference in value, also see if it is a single ply maple shell, it probably is but it's not a given.
Soundwise RKs are very distinctive, my first was a 50s 5 X 14 and it was very boxy, in fact to be honest I didn't like it.
A little while ago I bought a set of 1940 RKs and it has a 7 x 14 snare and it is one of my favourite drums, open and warm, just great. I don't know if it is general to find the smaller depths boxy it is just my experience.
As for value somebody from the U.S should answer that because here in the UK we pay a lot more for this kind of thing.
If the price is right grab it they are classics for a damn good reason