VIN HELP!! Need to know what month built.

Hello everyone,

I need to find out what month my 1971 Dodge Demon was built. VIN is LL29C1B104836. Reason is I need a gas cap and there are two available. Pre May and after May. Most of the gas caps I have found have 4 tangs and they don't work on my fuel filler tube. I think I need the two tang one with a very short neck. The ones I have found have around a 1/2" neck. Neck being the part with the tangs on it. I have seen a post on here that a guy posted a picture of the A-body gas caps by year. This was probably the best find yet for research. But trying to find the two tang short neck is almost impossible. When I bought my Demon it was a project car and didn't have a gas cap. Any help is greatly appreciated.