Well I certainly believe you about the Edward VIII cyphered SMLE because I've got one too.
I really know nothing about this weapon, but it did occur to me that sometimes small production runs of modified rifles were made for quasi-official organisations & so forth. If there are a few of these rifles all to the same spec., could they maybe have been a putative design for Park Rangers or some such organisation?? Somebody who needed the power of a 303 in a light rifle. It's pure speculation of course......
ATB