SAAMI specs were reverse engineered, based on SAAMI practice from the pre-existing .303 design specs from the 1910 with the adoption of the Mk VII ball ammo.. Those specs were "sealed" (cast in stone, not to be altered) by MOD WOPS #9 but SAAMI had to have things done to their idea of specifications.
If you look at the differences they relate to chamber dimensions & headspace with SAAMI's H/S tighter than the original British WD ones. SAAMI uses GO, NO GO & FIELD for gauging, War Office Plans & Specifications does not they just have a Min & a Max.
Its quite possible to "fail" a SAAMI spec "Field Gauge" because they do not use the 0.074" spec. While actually being in spec for the original MOD WOPS spec.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”