AdeE's thread 8 has pretty-well said it as it was! That Enfield supplied the rifle, tapped and threaded and stocked*. The other reason for Enfield doing this (since somneone asked.....) was because they had the necessary factory facilities to ensure that the mount holes were exactly at 12 0'clock in relation to the bore and aligned exactly linearly.
*So far as I am aware/informed by Major George Xxxxxs who oversaw the project, Enfield suppled the rifles fully stocked up with readily available from Ordnance converted fore-ends. Butts were one of the 'extra cost' choices as stated and if a variant was fitted by request, the original fitted one came back. They did come from SILE too! - as he gave me one that had been used during the tests, with a No8 butt plate fitted!