if the title and tag say it's a 51, why question that? if it has a 4spd in it, it's the SM420 no matter what the numbers on the case - you sure it's a V and not a poorly cast U?
the numbers don't mean much anyway cuz the facts don't always match the books - the SM420 was the only 4spd in every model for 51, and really the same in any model, except maybe a different rear shaft - RPO=Regular Production Option, meaning there were other trans options that year, ie: a 3spd
you might look for a date code on the trans case to see if it's Dec 51 or earlier to get an idea if it's been changed in the past 54 years
Bill
