L39, L42ish

[Thunderbox]

I hear your observation that it is 0.1" thicker overall than a standard No4 barrel, but from the photos it does still look like a standard Fazakerley .303 barrel that has had the front inch or so cut off.

In particuar, the knox form and chamber area look entirely normal - in proportion to the visible part of the receiver. Is the forend packed or bedded in any way? This often prevents handguards from being fitted, particularly if the reinforce is a bit too high.