I have a little difficulty with this. Is it being claimed that every No4 rifle sent for proof had a bolt head fitted that just closed over the .064 gauge? I rather doubt it. Perhaps a feeler gauge was used with the existing bolt head and the .064 guage and if "set back" of more than .003" occured, THEN the rifle failed proof?
I'm afraid I'm a bit sceptical about tests like this as well. Official "tests" have a funny way of producing the result that the authorities want, or that 'interested parties' want. (I've cited a few examples elsewhere concerning Canadian-produced equipment) We've been told here by informed parties, that the Indian No1 Rifles in 7.62mm are built of the same steel that UK
and Australian
No1 Rifles were built of. Of course, we've also been told that 'better' steel was used, but the weight of authority lies with the former opinion if I remember the parties involved.
Perhaps it's not your intention to suggest that bolthead and bodies were replaced on a regular, or even annual basis, but that's how it reads to me. Just thought I'd mention that.