Model 12 trench gun mag tube question

[mg42]

Hello all i have been looking at allot of ww2 model 12 trench guns and pictures.
Why do some mag tubes and receiver extension have the arrow alignment on them and some don't?

[mto7464]

I have seen one like this as well and was wondering this as well. Did you find an answer anywhere else?

[jim c 351]

I always thought that all M12s have alignment arrows.
When an arrow isn't visible, check to see if the tube isn't 180 degrees out of alignment.
Jim C