No separate retaining screws on mine. But I tapped out the 2 bolt holes to 5/16 UNC for easier plate removal.

I've only had pump failure to prime in a manual block once, using a new Metro oil pump.
I found the aftermarket manufacturer had failed to chamfer he corner of the pump. So it wouldn't seat on the gasket. Even using a thicker one I made.
Added a 1.5mm chamfer, all good. Beware new pumps!

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I wonder if the need for a screw to hold the adaptor plate was due to a loser fit in the pump recess, so some lost vacuum in the pump?

Has been a while since I posted my question. I can tell you that the cotton reel was for an auto block, I removed and replaced with a manual one. As far as the Spacer plate I tapped out two of the for holes for easy removal as I sealed the spacer on both sides. The result was great oil pressure, sits at just about 80 PSI on idle.

There was no counter sunk screw to hold the spacer plate in place. I suspect the well sealed gaskets and the oil pump itself has counteracted the need for this.

Anyway. Thanks to all that provided advice on this. Was doing my head in for months.

Now check the oil pressure relief valve...

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"Show me the Mini!"

Glad you got it sorted.
I've done 4 auto blocks, and still run 2 of them in the family.
Back years ago, they were considered to be door stops.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.