Identifying my 904

They did make a 4028878 non-lock up A-999 the same year, I remember the early '80's & how many folks despised those L/U trannys, it wouldn't surprise Me one bit if that thing was converted 35-40yrs ago. Back then there were a lot of good local trans guys, certainly capable of making the swap happen, I'd say if so You're already 5 steps ahead......
They also made A998s and standard duty 904s in the same year, as well as factory non-lockup A999s. Which is why I need some clarification on the particular trans ID if possible.
My catalog lists a single wire lockup solenoid for a 1987 transmission. As Killer6 says, count the spline teeth on the input shaft.
No lockup wire plug on this transmission, just the standard 3 prong NSS (I do have a legitimate LU A999/32RH to compare it to). Many of the (non-truck) lockups were fully hydraulic and had no lockout connection (most cars would not require a tow/haul switch).