I have a 6-51 HRA barrel that I believe is the earliest HRA barrel. It may well be a pre-production barrel as it is blued and doesn't have the lower band pin cut out. It was taken from a late HRA rifle (not be me) before it was generally known that HRA put a number of early barrels on late receivers. I'll post some pictures tonight.

I once saw a post in the GCAicon Journal for a 1-51 HRA barrel, but when I called the seller he noted that the posting was in error. If you have pictures of a 1-51 barrel please post them.