Well, the hard line has always been that there are approximately 50 of the 8 groove barrels, thus making that variation more valuable than the 6 groove....... 8)
the switch to 8 grooves was around 30xxx plus or minus, the 8 groove 6 1/2" guns are the most common type while the 4 5/8" 8 grooves are a little scarcer... but both 6- and 8-groove 10" guns are scarcer still..