occurs to me that maybe the initial pressure I had cold was limited not by the relief valve but by the viscous flow through that end play - in which case the set point of the relief valve could well be above 45 psi.
Bingo a bit of old Norton lore and factual wisdom is now yours.
If you read the appropriate section in the Service Notes you'll find the opinion that use of such a gauge is somewhere between a bad and terrible idea. Not only is it useless once the motor is warmed up, because most of the oil from the pump is doing its work on the bottom end and only a trickle is coming up to the rockers. If it breaks, you might not notice until you've lost your top end.
Crank/cover oil seals can not be blown out of their seat by a Norton oil pump with such easy exit of oil into the engine, UNless its a defective seal still being sold here and there but I don't know which or how to tell till too late.
Most tap pressure at the head but should read the same as a Tee in at the pump bypass. i'd prefer to get the worst news first so will put my gauge on rocker spindle.
I think you are now wasting time fussing with the supply side of oil pressure as its internal engine clearances that determine what PSI seen on the fly. Some see 50's at 70 mph, others nothing at 55 mph. Can try spring shims and see.