No.4 Mk.1 foreend with cut-out..

[Roger Payne]

I could well be wrong but I rather suspect that production lines would have been changed over to the later pattern as soon as it was practical & convenient to do so, & I would think that would have been most likely during the course of 1942. I took a look at a few of the early type fore ends in my garage & they are in the 20L's or lower (1942 mfr.).

Hope that helps.

ATB.