I don't think it is the 500/320 (aka 8x50R), as they had different sights, because of the Indian government's sighting & calibre restrictions, which is why the 303's weren't available there in the first place.

I've had a number of old #1 Pattern H.V.S.R.'s / Lee Speeds, especially the #1 (ribbed barrel) type, which had that large of a bore.

Look at the original 303 barrel specs. They were up to 0.321" in the grooves. (Land size 0.303"-0.3049" + 0.005"-0.0079" groove depth X 2).

I've got 13 of them now. Always room for 1 more of my favourite rifle, although I've sold a half-dozen in the last year or so that no longer appeal to me for one reason or another. Mostly the Pattern #1's (long ribbed barrelled ones), as they are pretty well useless where I hunt in the Canadian Shield, where 75 yards is a very long shot in the black spruce and the usual is 25 yards. I simply find the 21" barrel length VERY handy in the bush, so tend to really watch out for those.