there should be a small metal tag on the drivers door pillar above the dogleg with just the word CHEVROLET and below it a number - a 56 3100 would start 3A56XXXXXX and a 57 would be 3A57XXXX [or V3A if it was originally a V8 truck] .... the primary visual difference is the 55/6 hood is flat topped, the 57 has 2 'wind splits' sticking up - aside from the pronounced grill difference and different fender spears there's really no difference in those years .... and yes, a partial vin maight have been on the top of the frame rail by the steering box
Bill
