If it is authentic (there are vast quantities of forgery CSA stamps and covers out there) the stamps appear to be a Jeff Davis 10 cent stamp, Type I, which would make them Scott 11. These Jeff Davis portrait stamps were issued in 1863 and 1864 so that would make the date 1863 or later. The cancellation is so poor around the date that I don't think you could ever tell the year with any confidence unless the letter that was inside is still there and it has a date on it, or unless there is a dated backstamp. These stamps are not especially valuable since so many millions were printed and they sell on
ebay for modest amounts which makes it a toss up as to whether you would like to pay to have the cover expertised. Without an expertization I think that most collectors shy away because of so many forgeries out there.