The reason that I say that I'm pretty sure that they're the same is that the New Holland parts site only lists parts for one "manual reversing" transmission for the '65-'75 thousand series tractors, and it doesn't specify whether it's the 6x4 or 4x4, so I assume that they're identical except that the 4x4 doesn't have the extra forward gear in each range.
I just looked at it closer, and it appears that possibly the manual reverser was a 4x4 from 1/1/65 - 3/31/68, and it was a 6x4 from 4/1/68 and up. The part labelled "32" in the picture of the gears is listed twice. Once as:
So if the same gear is used for 2nd & 4th in one date range and 2nd & 5th for the other date range, it stands to reason that they changed the manual reversing transmission from a 4x4 to a 6x4 at that changeover date.
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