Hello, I just acquired this cover. By the looks of the instrument I would date it 1860s or 70s. However, I am not sure of the postcard itself which has a pre-printed oval 3-cent Washington. Any help dating this would be appreciated.
Your envelope is a "stamped envelope" or postal stationery entire. This embossed imprint was printed and released in 1864-65 (Scott U58), and the next design issues came out in 1870, so yours probably was used somewhere between 1864-1870. It could be used in the 1870s as well, but was most likely earlier.