Ughh, doesn"t sound like a whole lot of fun.
Here"s what I was able to find from the Case/IH website on the different part#"s.
I"m somewhat surpised that a difference of 3 teeth can make such a difference.
VAI Differential gears;
2 VT3725 Bull GEAR - final drive, 63 teeth 2 VTA-2514 Diff Brake Gear — side, with bushings, 16 teeth (tractors before No. 5555000)
2 VTA2814 Diff Brake GEAR - side, with plug, 16 teeth (Tractors after no. 5554999) 1 VT3421 Ring GEAR, 43 teeth 1 VT3410 COUNTERSHAFT AND BEVEL DRIVE PINION
VA / VAC Differential Gears;
2 VT3422 Bull GEAR - final drive (66T) (44" - 80" tread on VAC)
2 VT3606 Bull GEAR - final drive (65T), Used on special mowing VAC 13 tractors with adjustable front axle after #5570000. 2 VT5467 Bull GEAR - final drive (67T) VAC 44" - 88" tread and VAO 15 (VAC, VAO) 1 VTA587 RING GEAR AND CENTER WHEEL ASSEMBLY W/SHAFT (Includes VT3457 shaft) (No teeth count listed?)
2 VTA2512 Diff Brake GEAR - side (Includes VT3479 plug and 2-VT3765 bushings) Tractors w/disc type brakes
1 VT3410 COUNTERSHAFT AND BEVEL DRIVE PINION