That clears that issue up, but what was going on that the "shoulder" needed adusting?

Was it something to do with providing clearance for "mud" or dubious wartime ammo? Being a rimmed round, you can have a "generous" shoulder, and even body clearance (instant "improved" cases), and no problems with headspace / striker protrusion, unlike a rimless model in which length from head to shoulder datum is critical..

As a side note, what is the source of that illustration?

Back to exploring the crypt for the various .303 chamber drawings.

Does anyone have barrel / chamber drawings for Bren and Vickers, as well.

There seems to have been a lot of creative engineering in these regions for a long time, but even drawings for 7.62 NATO barrels show subtle differences between machine gun and rifle chambers.