My recollection is that the Parker Hale one piece mount was stated in the (1931?) catalogue to have "been designed by us during the war"; the implication was that they submitted it - one assumes - but it was not accepted for service. Probably because it required removing the standard backsight and inserting the rail into the sight base (without any permanent alterations) and relied on a hardened screw with a sharp tip digging into the Nock's Form. Such innovations no doubt being unwelcome!
IIRC the distance between the bases on the Winchester A5 was seven inches, so one can easily tell if the standard two piece 3/8" dovetail blocks used on the Ross Mk.III for example, would fit on the SMLE or if the backsight base would obstruct. I'm not going to bother checking, but I wouldnt' be surprised if they do.
One of the good points of the Winchester mounts when used overhead was that the scope which slid forward with every shot, could also be slid forward to clear the action for clip loading. Return springs for such scopes apparently didn't come along until after WWI.