['Early machines have an oil tell tale button either on the side of the engine or in the tank panel which must be activated by a certain pressure, I wonder what pressure that was? ...']

The same or less than the anti drain valve I'd have thought..(7psi)..
Nearly all makes dispensed with tell tales pretty quickly, as far as I know only Triumph persisted post war...Ariel removed tank panel pressure gauges due to the high number of worried owners returning to dealers because they thought the oil pressure was too low (It probably was less than they expected at least)...These roller bearing engines register virtually nothing pressure wise at tick over when hot....I fitted a pressure gauge on my (white metal big end) Ariel Square Four and even that ran at 60psi when cold and about 20psi or less when hot at tick over...

Once the gauges were removed everyone was happy in their ignorance....It's debatable whether tell tales are very useful either really...If your not actually looking at it when you have a pump/drive failure it'll be the grinding and knocking of tortured metal or the seizure that will let you know what's going on...Ian

email (option): ian@wright52.plus.com

As a matter of interest here's the result of some pressure tests on BSA M/B Series pumps (averaged)....The pumps were running 'dead headed' or against a 'blind gallery' to obtain these figures at different revs. Note the average maximum pressure of 60psi...Of course the pump wouldn't do that against an open roller big end...Ian

email (option): ian@wright52.plus.com