Um, probably not actually. Two reasons:
That envalope is far too pristine to be 155-160 years old, so maybe a reproduction? But a poor one for the second reason:
That's not the US flag from the Civil War. There were four US flags from 1859 to 1865 with 33, 34, 35 and 36 stars. The blue fields above are frankly a mess and have at best 24-28 stars. 24 stars dates to 1822 and 28 in 1846. 39 and 15 years prior, respectively.
That envalope is far too pristine to be 155-160 years old, so maybe a reproduction? But a poor one for the second reason:
That's not the US flag from the Civil War. There were four US flags from 1859 to 1865 with 33, 34, 35 and 36 stars. The blue fields above are frankly a mess and have at best 24-28 stars. 24 stars dates to 1822 and 28 in 1846. 39 and 15 years prior, respectively.