[FoxChassis]

Triple letters were used from 1987 through 1989.
Single letter was used from 1990 through 1993.

For 'aero' SSPs I have so far seen the following:

1987-89 = XXX
1990-91 = X
1992 = A, D, F, G, H, I, J, K, L
1993 = A, C, D, E, F, G, H