Very nice photos. I agree that they appear to be the same man.
I am fairly confident that this is one of the two Kiehn brothers.
The second picture, in which he appears younger and has the EK2 in the buttonhole was likely taken not long after (or to replicate) the award of the EK.
On the day it was awarded, the second class was worn in the buttonhole as shown. The uniform is clearly that of a "Train" (Brits would call it Army service corps). The light (blue) piping on the front edge of the tunic, cuffs and collar, as well as the two horizontal buttons on the cuffs mark this as such a uniform.
The first picture shows him after he had left the army. I found a note that one of these two left the army on 13.07.1872 and was given the rank of Rittmeister (captain) and permission to wear the "Landwehr Armee Uniform."
His medals are, from left to right
campaign medal for 1870/71
campaign medal for 1866
The two officers were promoted to 2nd lieutenant of the Landwehr cavalry on 12.03.1859 and 20.09.1859 and both were assigned to the Landwehr cavalry 1. levy of 2nd Battalion of Landwehr Regiment 14 in Bromberg.
As you may know, officers of the Landwehr were not active duty. The Landwehr formed a sort of home guard and only called up for an annual training period and in the event of war, as was the case in 1866 and 1870.
The only thing about all of this that bothers me a bit is that while they were assigned to a Landwehr district in West Prussia, they served in units assigned to a corps far from there, in what was, in 1870 the IX. Army Corps, HQ in Altona. The locations of the two photo studios were both in IX. Army Corps area.
That said, nice to see that the photos have survived all these years and that the family has taken an interest in them.