A bit of information on the helmet itself: It is a private purchase Prussian infantry officer style helmet. Your ancestor bought this helmet himself, it was not issued to him by the government. It is infantry due to the flat chin scales, mounted troops have curved scales. It has the officer silk and calf skin liner. The serrated discs on each side of the helmet are called cockardes/ cockades. These are the officer style 2 piece construction, a silver ring fastened to a painted metal disc. The colours of the disc have significance, the black and white are the state colours of Prussia. This cockade is always on the left side, left ear of the soldier. Above the right ear we have the Reich’s cockade, red black and white which are the colours of the German Empire. The Reich cockade was not introduced until 1897, so this gives an approximate date for purchase, sometime after this year.
The spike is also officer style with the egg and dart shaped ring around the spike neck. The cone of the spike may unscrew, so that a horse hair parade plume could be put on the helmet for parades. There are 2 characteristics of the helmet which are not officer which account for our members saying your ancestor was an “Unterofficer with Portepee “ . This is a unique German rank.....think senior sargeant entitled to wear an officer sword and sword knot (portepee). You can see that he is quite proud of this from his pose in your photo, that sword and knot are right out front. A true officer helmet would have brass stars holding the spike base, yours has domed studs. The eagle’s crown on the helmet plate would be pierced on an officer helmet. We can’t tell from your pictures. Domed studs and a solid crown are ordinary ranks characteristics which again indicate you ancestor was not quite full officer rank. I hope this helps and that other members can provide more specific information on your relative.