After all this time reading posts here extolling the benefits of kerosene stoves, I saw a Primus 51 for $15 and figured why not give it a try. Definitely need fettling, and it is missing the flame ring and the legs, but I think it is in pretty nice condition... Should be a fun little project!
Great find! You can make new legs easy enough. Fettlebox has all the gaskets you need, they usually have flame rings too but are out of stock at the moment. There is a fellow selling reproduction flame rings on ebay right now.
I picked up one exactly like this 10 years ago. Tank and burner only and missing the legs and flame ring. Mine has a AU date code on the bottom signifying 1955. As far as I know Primus 51's with this style of venting, with airtube with bleed screw and flat filler cap were only used a few years in the 1950's.