General TR Overdrive Side plate

The side plate on a Type A overdrive compresses the long heavy springs inside the overdrive/piston unit. On the back of that plate there is a machined depression or “hole“, and I am wondering whether a pin or keeper must be inserted in that hole so the springs are centered and compress squarely to the side plate. Can anyone please shed some light on whether this is necessary?

There is supposed to be a steel pin in that hole, which fits into the center of the inner spring.

I can't say how "necessary" it is, but I doubt Laycock would have gone to all that trouble if they hadn't felt it was important. The design does have a certain Rube Goldberg quality to it, but every piece plays a part.

WOW, thanks Randall. I should've known. But (and I know not all questions have an answer) my new end plate from Moss has no fitting for that hole, and I see none in the catalogue.

Yeah, it's not listed as a separate piece even in the factory catalog. But the plate (55 in your diagram) is described as an "assembly", which I suspect means the pin was part of it.
The later IRS overdrives didn't have the inner spring (and a much weaker outer spring), so the pin may have been deleted later on. I'm sure it added some small cost to the unit. The part number didn't change, though, so maybe Moss overlooked the difference? Or maybe the factory decided the pin wasn't needed. I don't know; just guessing.

But all of my A-types have been the large accumulator variety (for solid axle cars), and they've all had that pin.

In my early "A" overdrive there is only a single spring with no dowel to center it. The casing slot for the spring is all that keeps it centered on the plate. There is also no hole to accept a dowel.

That's interesting. I didn't know the early units lacked the inner spring. (No mention of that in the SPC.) Sounds like they added the peg when they added the spring, then took it away again when the inner spring went away.