Any pictures ? When you say "'55, '56, or '57, with earlier grille", you have me curious. 2nd series 55, '56, & '57 model Medium Duty trucks all had the same grille unless they were a "Fleet" model, with the expanded metal type grille.
Sorry, I had brain freeze when I typed that. For some reason I was thinking of my '55, '56 or '57 half ton that I got from Barrett Jackson that they sold with a phony VIN. It is possible that it is a '57 with an earlier grille.
http://www.barrett-jackson.com/Archive/Event/Item/1955-CHEVROLET-3100-PICKUP-116374If you run that VIN, it comes back as a '63 1-ton.
Barrett Jackson says "Buyer Beware".
Truck has no numbers on the frame so I don't know what it is exactly.
Of course, the 55-57 Big Bolts all have the same grille. I've read however that the '57 have larger front wheel openings?
