I also have done business in the past with 2curios and have been well satisfied. They do offer some high-end repros, but are careful to identify them. You will note that the posting for this particular Gulf Supreme sign stated NOT REPRODUCTION in all caps.

Here's what they had to say when I inquired:

<<George has returned and we looked over the "discussion board" link you sent us. The best that I can tell you, is that we bought the sign as old and original from a collector. We describe items as to the best of our knowledge, and we do also strive to keep our customers happy. We do not claim to be experts, we just like this stuff! There is no real way for us to know. I just don't know how one "verifies" the authenticity or non-authenticity of an item. I know that may not be much of an answer, but as with all dealers, just can't always know... to post this at the discussion boards.>>

I'd also have to agree with the posting that notes that different companies made variations on basic designs over a period of time. This is particularly maddening for the Ford collectors out there as FMC used the same designs over and over for many years, e.g., Genuine Ford parts ovals. I can think of 5 variations off hand and all are regularly traded and accepted as originals.