The 6 inch Model 66 dash nothings guns very probably had original shorter 4 inch barrels replaced with 6 inchers post-purchase. At the time, ANY Model 66 was hard to find. The factory would do swaps and there were a surprising number of loose 6 inch barrels floating around after they had been in production for a few years.

One of my coworkers found a 6 inch barrel and our department "armorer" installed it, clamping them in unprotected vise jaws. Coworker was pissed but the "armorer" was a lieutenant, so what could he do?