Is This Arisaka 1943 or earlier?

[Patrick Chadwick]

He also "carved" "screwdrivered" his initials in the buttplate, but why twice?

I am not convinced that those letters are hand-carved. The strokes of the W and M are accurately parallel, and the M looks well-nigh identical to the W if you reverse the photo. I think it is some kind of unit/ownership mark, which was applied a second time (the horizontal version in the photo) because the first one was poorly struck or not in the correct orientation.