Bob,
we can narrow him down to four possibilities from 1899 (when the guard eagle was introduced) until 1914. He wears a Johanniter Ehrenritterkreuz at the throat. He was not one of the regimental commanders during this period and presumably one of the following four UR 13 field officers who held this decoration during this time frame.
Major von Alten
Major von Uechtritz und Steinkirch
Major von der Wense
Major Freiherr Knigge.
I will check to see if I have images of any of the gentlemen this evening to try and narrow the search.
Regards
Glenn