My 1955 1st front and rear bumpers have cast letters (typically GM-D, GM-U, GM-B) on the inside. My guess is that the letters help identify a particular GM foundry. I don't know if 1957 bumpers have these numbers but it is the kind of information you need to know if you are truly looking for originality. My bumpers are spring steel and will break or crack before they significantly bend. Reproduction bumpers, as far as I know, do not have the cast letters and are made of metal that can bend without breaking. Most come from Asia.
"Mint" means new and unused from the day Chevrolet made them (in your case 1957). Today's distortion of the original meaning of "Mint" is something that looks good.
Excellent quality and condition comes at excellent prices. Something that Jay Leno is willing and able to spend.
I would start by examining what you have for originality. If what you have is original, undented or mildly dented and no severe rust pits then you need to locate a first class chroming business that does show-quality. The number of these business are declining in the USA because chrome plating can be an environmental hazard.
