Officers, Organized Militia of Delaware, ca.1880

A picture I found of some officers from what I think was then known as the Regiment of Infantry, a unit from the Organized Militia of Delaware. Note the pickelhaube.

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A nice photograph,
As far as I’m aware, the 1872 Regulations cavalry/mounted men received the plumed helmet.  
Foot soldiers and their officers wore the shako for dress that was  piped in the corps colour for the enlisted men and in gold for officers. The shakos were worn until 1881 when they were replaced by the dress helmet with a spike on top.
The small hunting horn insignia of the Infantry was replaced in 1875 by the crossed rifles. These were made of gold bullion for officers and stamped brass for the enlisted men.
The hunting horn insignia was to remain in service as the insignia for the musicians.
It looks like there kepi insignia is crossed rifles so this would place them post 1875.
IMAGES; 1 United States Infantry-Illustrated History 1775-1918-Urwin 1988, 2 & 3 The Horse Soldiers 1776-1943 - Randy Steffen.
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I think the offficer in the helmet may be a US Regular Army advisor/rater attached to the regiment during summer encampment.