That sight, thread 13 is for the standard No8 rifle. The H is for harmonisation - when using landscape targets. It was a short lived thying because while the bullets were meant to hit the landscape target 28"(?) above the point of aim. actually aiming at the target using an eyepiece that high up on the sight was purgatory. Jeeees, the targets were cheap paper so the sight and harmonisation fell into disuse and by 1958 had been deleted from use. Thereafter you just shot at the target and pasted it up. The landscape targets lasted for ages in any case. Another of those good in theory - crap in practice ideas that went by the board.
Anyone got a landscape target to illustrate the point. There were several versions including sniper landscapes, open countryside, towns, villages etc etc, old ladies pushing prams up the village street - who ALWAYS got shredded as I recall!