I believe that this is a perfectly legit Oldenburg enlisted private purchase helmet. It has a standard officers liner and an officers rear spine. The rear spine is somewhat unusual. It seems that most enlisted private purchase helmets have the non-vented, smooth, rear spine instead of the officers type. The fittings were up to the purchaser, so this configuration is 100% acceptable. The spike is an earlier model 1867/71 type, but they continued in use for private purchase helmets even into the 1900's. These spikes only had two vent holes.
I am not sure why you say that this is a model 1891. Model 1891 was an issued model that incorporated the new M91 chinstrap mounting lug. That model also included a vented spike that could be opened and closed (not always though), thicker leather than the model 1895, and a non-vented spine. This private purchase helmet could be as early as 1891, but I would guess it's later. It has a Reichskokarde which was not introduced until 1897.
Very nice helmet with a rare Wappen! Congratulations.
John
