the ST-225 drive uses a similar circuit but it is an older design and less integrated. the ST-251's SSI257.2 chip drives the center taps of the head windings. they just "garbage collected" the transistors, head select decoder, and resistors driving it along with the transistors that set the write current.
by looking at the ST-412 schematic, i figured out that the ST-251's SSI257 chip contains a bunch of steering diodes as well as an op amp (an NE592 equivalent).