are you sure there aren't any signs the rifle spent any time in finland? That looks to be the finn-added 200-meter notch and the corresponding flat on the barrel shank allows for proper clearance when the 200-meter notch is used.
After I read this thread, tplan, I remembered the rear sight had been modified on my 1916 PTG Instructie, #23189. Looks like all the steps have been lowered, (esp the lowest one) and the slide has been worked on the bottom to make it thinner. The receiver has only a slight relief on it for clearance when the slide is on the lowest step. I can't see anything that would point to this rifle having been in Finnish hands, other that it has been counterbored?
This is an interesting topic and one that I have been wanting to dig into for quite some time now. I feel that the subject of this topic should be researched thoroughly.
There are quite a few M91 rear site modifications not associated with the Finns. This immediately brings up a few questions.
Who did it?
That is the million dollar question. Since we don't have documentation available showing a definitive "who done it". Let's see what possibilities exist.
Russian/Soviet armorer's (Primary and Divisional), Balkan region armorer's, soldiers in the field.
When was it done?
Again, no documentation. An answer to the first question would allow for a better guess.
Where was it done?
This question would very likely get answered by the 1st question.
Why was it done?
This one seems to be fairly simple. To get the rifle sighted in properly. If done in the Balkans. The standards could have been quite different from the original Russian/Soviet standards.
What are all the different modification possibilities?
This one can get difficult due the infinite seeming possibilities. Each rifle may and most likely, would have required a combination of different modifications. I have also noticed that there are 2 different rear sight bases. This could lead to different modifications being applied.
I believe that all the modifications are related to getting the rifle sighted in properly. I haven't had the time to research the subject like I want to do though.
I believe the 6th notch was added due the rifle shooting to high. By lowering the rear sight. The barrel has to be rotated downward to lineup the sights. This lowers the point of aim. In many cases this also required that a beveled area be filed from the barrel shank to allow the rear sight to sit low enough.
6th Notch added to base.
The following pics show the differences in the rear sight heights, from original thru the different modifications. Notice the beveled area on the barrel shank as well.
There were also rifles that would shoot low in regards to point of aim. Another modification seen is the addition of a dovetailed shim being installed into the rear sight spring guide and peened into position. This would casue the rear sight to sit higher when in the lowest position. This causes the barrel to be rotated upward to lineup the sights and raises the point of aim.
The following pics are of the shim that was added at the rear of the sight spring guide. Unfortunately I don't have pics of this shim installed. My camera died during this bit of research and I haven't had time to get back into it. Too many interesting projects and not enough time.
There are a multitude of minor modifications made to the bottom of the M91 rear sight slide that would have been used with each of the above modifications to the rear sight base. Here are a few examples:
Unmodified rear sight slide.
Maybe someone here can continue the research where I have left off. With my current list of projects it will be quite sometime before I can dig deep into this. I would really like to see more on this topic. So, who out there needs a research project? This would be a good one!