Not quite correct smelly Mk5. The cut-off block was deleted for the No5 and the late No4 Mk2 rifles. I would imagine that the block was retained in No4 production not because there was any thought of retro modifyiong the rifles to take one - we'd all moved on a tad by then. But because it was an integral part of the structural design and to eliminate if from production was more trouble and expense than could be justified. Just my opinion of course