This seal sits in the gearbox -07/60 on the flywheel side and ensures that the oil cannot leak along the primary shaft.

The larger collar ensures that you cannot fit the seal too deep, as the split case gearboxes do not yet have an inner rim for the correct depth as with the later ones. The later model seal BBT # 1416 also fits, but you could push it too far, causing it to start leaking. Correct high quality parts for a perfect fit.