The rear axle number is stamped on the front side of the left hand axle tube. I found mine just inboard from the rebound strap, about over the exhaust pipe, and just above the weld line in the axle tube. This one says "11586 10/43". The first numbers are the axle serial number. The last numbers are the (original) gear ratio. The rear axle is generally quite robust and durable in the MGA, but a common reason for changing it is to reconfigure from disk wheels to wire wheels or vise versa. The wire wheel axle housing and half shafts are about 7/8" shorter at each side to allow for the length of the splined hub. Wire wheels may be preferred for appearance. Bolt on wheels may be preferred for the availability of more options for competition or appearance.