This 900a Series is in to get TRS jacks in to replace the terminal connectors. I made a Custom PCB to replace the board mount terminal connector and reuse the original metal work with a 3D printed mounting plate.
Ah-ha, the old TO-127 package! It looks like the middle 4 Darlington transistors are in this package mating with the 0.156" connectors. The outer two regulators are conventional TO-220, going directly into 0.1" sockets.
Easy to take apart, but it's a very unreliable way of connecting to power device leads. Any increase in contact resistance through ageing can quickly build into a run-away failure.