British pressure and a soup sandwich

[Parashooter]

Ed, you'll probably not find any reference to a UK change from the base crusher system. As far as I know, they're still using it for some of the older cartridges, just as the radial crusher system is still sometimes used in the USA. It's one of those "ain't broke, don't fix" deals. Note this from the website you referenced a few posts up -

In Britain, a third set of crusher standards were developed, using a "base" crusher. The crusher was a short, thick tube placed behind a piston at the base of the cartridge, and the firing pin passed through the center. The cartridge case was well oiled before firing, to minimize cling to the chamber walls (if not oiled, the indicated pressures were about 25% lower). To prevent case rupture on set back of the base, the crusher was first deformed in a press to a pressure a bit lower than that expected in firing. The units were generally stated in British long tons per square inch, or tsi. Pressures indicated by this method run 10 to 20% below those indicated by radial crushers. Kynamco in England still rates their production cartridges with this method.

My comment was in reference to the Australian pressures you quoted -

In Skennertons “Australian SMLE Variations” I saw that the older Australian cordite .303 round was 19.07 t.s.i = 42,720 PSI ? and this was later increased for later Mk.VII rounds to 20.71 t.s.i = 46,390 PSI?

Those look like radial-system figures to me. As I'm sure you know, Lithgow was set up with Pratt & Whitney tooling from Connecticut. Maybe radial-system pressure-test equipment was included.

One can get very frustrated trying to nail down pressure numbers to the last trivial ton or two. A variation up to around 15% in pressure results from carefully-made "reference ammunition" fired in the same pressure gun is apparently considered normal (Handloading, NRA, p.128).