I believe the testing was done on the plate at the manufacturer, not once the plate was cut up into hull sections, or after it was assembled. The test was specific to ABP3 (Australian Bullet Proof No.3) plate, as this type of plate manufacture/formula was unique to Australia. As far as I can tell from the limited info I've located so far, the testing was done with a test rig and was supposed to simulate a bullet strike by service Ball ammunition, but did not actually use a ball projectile. I've been unable to find any diagrams or descriptions of the test rig, but it was built so the testing was repeatable and consistent, ie was a valid comparative measure of the brittleness and armour qualities of each ABP3 plate. You will find that all the marks - which I agree do not look anywhere like a Ball round impact at 2500-odd fps - are the same and show a flat 'punch' mark, rather than exertion of steady pressure or the impact of a pointed bullet.
ABP3 had a tendency to become very brittle if the final quenching process was done at a slightly varied temperature - early Cruiser tank hulls consistently failed the test because of the difficulties of maintaining the correct temp across such a large structure, whereas flat plate was comparatively easier to control.
ABP3 is unique to Aust armoured vehicles, so I doubt that the exact same rig with a captive pin was used in the UK or elsewhere. Certainly, it appears to be a variation on the standard test method, with pressure, speed of application etc tailored to test the qualities of ABP3, particularly brittleness.
Mike C
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