This is only a rule of thumb & no doubt someone will contradict me (!), but in general beech was used for much of the stocking up of early run of production No4's (1941 & 42 mfr), though by 1943 kiln dried American Black Walnut was in common use - at least on BSA made rifles. I have seen plenty of early UKmade rifles of all three manufacturers wearing beech clothes, but by now it is usually quite darkened with oxidation & dirt/grime impregnation. I often have to look twice to check it isn't walnut. Of course this does not apply to Trials rifles which were originally all stocked up in walnut, but I have two 1941 BSA & two 1941 Maltby 4T's & all four are stocked in beech.
ATB