You are correct about the No. 4s, those of us not hip deep in Britishrifles err in calling them all SMLEs. I have been in touch with G&H about those mounts (bases). While they are no longer made, it appears from Skennerton
that a std G&H flat side base would work fine as neither eye relief or bolt lift should be an issue.
As for the Lyman/1093A3/4 connection, Campbells book specifically mentions Lymans on 03/4s in Korea. No doubt some of the same ones seen in Vietnam.
Both the Navy and USMC held on to scoped A3/4 rifles for shipboard duty long after they were phased out of shore duty. Good for mine destruction and dealing with the far too common in/under water sappers in Vietnam.