There is a possibility that the 75 had 6 grooves. Winchester used that rifling on the Model 52 and some other higher end models. The 69 was cut with 4 grooves as were other less expensive models. The rifle twist was 1 in 16 in all rifles chambered for 22 LR, then and now. The 22 short had either a 1 in 20 or 1 in 24 inch twist. This info came from Phil Sharpe's "Rifle in America" published in 1938. The Model 75 wasn't in it, nor was the M-69A.
I expect the Model 75 was made to tighter tolerances and with more care in either the Sporter model or the Target model. It has been remarked on that some Sporter 75's had better wood on them than the 52's being produced.
Apologies to salarguy for running off with his thread.
