I believe this Robert McMaster is the brother of my great-great grandfather, Alexander McMaster.
I have in my possession a letter that I copied that Robert wrote to Alexander in I think around 1864 (I will get my genealogy out this weekend.If memory serves me right, Robert's wife was referred to as Catherine.
Alexander was born in Maryland in I believe 1828.He ended up passing away in the Eugene, Lane, Oregon area and he is buried in Lowell, Lane, Oregon.
In this particular letter there were a couple of family names mentioned.(I will pick up my genealogy from an Internet cousin's this weekend).
Hope this works out that your Robert and my Alexander are related.