"Maybe they wanted a 6" bull barrel which Ruger never made. Or maybe there was damage to the barrel or bore."
A 6" barrel seems like a waste of a good 10" gun. Especially when Ruger built the 5-1/2" bull barreled gun.
If my faded memory is correct, the 5-1/2" guns were done to have the guns allowed in some form of competition way back when.
Yet,, in some states,, hunting,, a 6" barrel minimum length was required. I have a 5-1/2" bull barreled MKI. Decades ago,, I had a very lively discussion with a game warden about the barrel length & hunting on game lands. Back then, a 6" barrel or longer was required on game lands,, yet on private land,, no minimum length was required. I was trying to get the warden to explain the idiocy of such a requirement when my barrel was 5-1/2" and obviously a good hunting handgun. Exasperated, he finally said to me; " If I see you hunting with that gun on game lands, I won't write you a ticket. I know how you shoot & all." I really didn't hunt much on game lands back then, but I was trying to understand the law, and he & I were enjoying discussing the way laws were worded.