If you have a diff already removed, then count the teeth. Just the pinion would do.
The 3.9 to 1 ratio has 35/9 ringear/ pinion teeth for an actual ratio of 3,889 to 1 but it's widely known a 3.9 t0 1.
The 3.7 to 1 diff has a tooth count of 37/10.

If NZ B310 wagons had the A12, they would have been fitted with the 3.9 diff & would have been down on performance compared to the lighter sedans & the earlier A12 powered cars which were also lighter.