some of the very early 70's T/C's had a slow twist barrel that is different from the current offerd 1-48 or 1-66. If memory serves right it was 1-60 :roll:(as if I have memory) and the grooves were deeper.
common problem with measuring bore twist is not enough patch on the jag for a snug fit and/or a loose jag on the rod.
Double check them measures Keith,,cause 51 just don't sound right,,, heck just pull 12"s,put tape on the rod and tape on the barrel and make yer marks,,then you snug thet jag down with a plier if need too,,cause a clockwise twist WILL unscrew a clockwise thread when pulled. The rod will pull further than the jag,,because it's unscrewing itself as you pull. If it's not "tight" it will slip or skip the rifling