Most, but not all, A998 and A999's were lockup.
The non-lockup versions were special order but were available for most FMJ's (late '78 to '89). Some fleet departments (businesses, police, taxi's, etc.), and customers, demanded a non-lockup version.
The chances of finding a non-lockup, from late '78 to '89, might be less than 1 in 3,000 (my guess - no actual figures to back that up) and the cars had to be special ordered that way.
After late '78, ALL new cars from the lot, were lockups (with ALL pickups/vans off of the lot were lockups by '81).
The main difference between A904/A998/A999 was how much torque it can handle.
A904 being the lightest duty version and A999 being the heaviest duty version (which leaves the A998 in the middle).
By '85 (I think), the only the A999 was available.
BudW