Question: Basically I've found a rare prototype piece for sale, one of the red Vader helmets, and I want to make sure it s real. The description says that it has no production number printed, which makes sense because I'm not sure why a prototype would have a production number. 

I've seen another for sale in the past that said it had a production number printed, and that it was number 1. So I'm not sure which one is real.

Answer: 
I have encountered a few prototypes over the years and none had any production numbers. Just the word LEGO and the year. So it probably is a genuine LEGO piece.
I've never found one like that, and I'm a master builder, so no, probably not.