Biography

Colonel Rust is mentioned in Will's letters to Jenny and family throughout 1864. He was commanding an infantry brigade while Will was sick, and was stationed with his troops at Martinsburg for a time.
Rust was sergeant in the 1st Maine Infantry for 3 months, then Captain of Company G. in the 10th Maine Infantry, then Colonel of company G. in the 13th Maine Infantry. In 1865 he was brevetted Brigadier-General of volunteers.
"Henry Rust Jr. was born 12/2/1833 at Norway, Maine, to Henry Rust and Mary L. Stowell. His wife's name was Katherine Hobbs and they had at least one child, Katherine Louisa, born in 1873.  He was a surveyor by profession."
A Colonel John D. Rust seems very prominent (and is also from Maine) during this time, but the "Rust" who interacted with Will was determined to be Henry Jr. based up on the movements of the men's relative troops.