I think we all have to assume that one (or more) of the words has been mis-transcribed. As
@Story points out, the details for this being the 7th KY are pretty solid. so I'd eliminate that as the error. The possibility that the company is wrong is there, but the fact that there aren't other Depews in other companies makes it harder. The date is also something that could be wrong - is it Jan or Jun?
I think we seriously have to consider that the Depew is the error. There is always a tendency for people transcribing hard to read documents to find what they are looking for - so if the original transcriber started with the thought that this was written by a Depew family member they may well have seen Depew when it wasn't there. We know there was damage to that area of the letter, which is why the first name is unknown. It is possible that both parts of the name were illegible but the temptation to see Depew was strong enough to align the letters in the transcriber's sight.
One argument for this is - there is a lieutenant from Company F who died in 1863 after being wounded at Vicksburg. Thomas Buchanan fits a lot of the boxes - not only the rank and company, he was also old enough to be married and have children. I have not been able to find enough records for him to ID his sisters. It may be that he had some times to the Depew family which caused his letter to end up with the Bible.
@Kwms do you see a connection in your tree?