Pinned rear sight elevation slide on a Long Lee

[enbloc8]

I just acquired an 1896 Magazine Lee Enfield, made by BSA, that spent time in New Zealand before coming to the US. While examining it, I noticed that the elevation slide on the rear sight was pinned in place, so that it's fixed for about 300 yards lowered and about 1,200 yards raised. Was this a legitimate service alteration, and if so, what was the purpose?