Edit, more precisely, between '58 (when everyone started making cantilevers) to '62 (last year for Evans). I would guess in the middle because that isn't the early guard, and those aren't the later forks.
I believe that the serial numbers may indicate the year of manufacture.
Evans may have started with 1954-A(?).
A boys 26” might be stamped “B26”.
I have heard that in later years, Evans filled orders with Murray built frames (stamped with an “M”).