HP Paintjet stepper motor driver IC

I've got an HP Paintjet printer on the bench (this is on-topic, it seems to have been made in 1986 or so). I've figured out most of it (even the PSU section with a complex custom DIL resistor pack), but I can't indentify the driver chip for the linefeed motor, which is a 4-input unipolar stepper motor. Anyway, the driver chip is marked 1820-4864 (too late for my equivalents list). It would seem to be a quad power driver chip rather than a stepprr motor controller, as it takes in 4 bits from one of the microocntroller ports. The pins, as I've traced them seem to be : 1 Input 2 +5V 3 Output 4 N/C 5 Input 6 +5V 7 Output 8 Ground 9 Clamp 10 Output 11 Input 12 +5V 13 Output 14 Input 15 +5V 16 +5V It's a normal 16 pin DIL package (no heatsink tabs or anything). I usspect, without prood, that it's actually 4 separate stages, AND or NAND functionality, : 1 & 2 -> 3, 5 * 6 -> 7, 12 & 11 -> 10, 15 & 14 -> 13 Does that pinout ring any bells with anyone? It's not any of the chips in my TI Interface Databook.. -tony