Good deal. I should have said: Anti-Lock Brake controller.
Mine on my 1990 G30 triggers a check engine light when plugged in.
Probably a part number, I'd guess they are matched up to the weight class and brake system indicated in the VIN number. So i'd be looking for one matching my 2GAGG VIN. If I ever find one here. If I ever feel like fixing it. It stops fine.
Haven't seen one new, I have seen units on EBay. I'll pull one off of a G30 in the Yard here if a one ton ever shows up.
Good Wrenching, and Van On!