Every Memorial Day we read off all the names of the veterans buried in the Corvallis (Montana)cemetery. I'm going through the list and putting the veterans in groups according to their time of service.
I came across two gravestones that I believe belong to Civil War veterans. I was hoping someone could confirm or deny my theory.
The two veterans are:

Peter Friend and WH H.H. Friend

On Peter's gravestone it says:
Co. M, 73 EN MO Mil

On WH's gravestone it says:
Co A., 4 MO S.M. Cav.

Again I think that these were Civil War units from the state of Missouri.
Any help would be greatly appreciated.
Thank You,
Doug Mason