The No4 rifle is only hardened in two places and that is induction hardened on the locking cams of the body. Even with the somewhat archaic (by modern standards) hardening methods, it would seem rather unikely that this process would have extended to beyond where it was necessary. In any case, even if it was hard, it would be a simple matter to drill through and tap the hardened area at the rear. And in any case, just thinking out aloud, how would they know how hard it was until they tried it?

LB wasn't short of rifles for the sniper programme, they were short of the wherewithall to get the programme moving. That's why H&H got involved