I do not know the transition, but at some point, I would guess in the 30's, they transitioned from the roller coaster to the tangent style sight. I would suspect shortly after the rearmament program began.
I cannot find my references right now, but I believe Mauser made several thousand tangent rear sight bases for the Haenel rear sight that was used on the Whermannsgewehr. If I can find that information again, it would give a good reference as to the transition point.
Haenel rifles seem to be a mix of the sights, but I have a very small sample. It could be possible you could order the new tangent, or the old lange for less cost, but I have not been able to find a catalog or documentation to support that.
If you are interested in selling the barrel, please send me a PM.
Thanks