The visa is a SPOUSE/CP- Spouse of......
It is valid from 05/11/14 to 05/08/17.
I included a letter from HR stating my employment details which included that I am enlisted until 06/01/2026.
Hmm, okay - well, it should have been issued for 5 years (though this has now been changed to 4 years, looking at current guidance), but instead it was issued for 33 months, which is the normal length of a non-HM Forces spousal visa.
So, I have a feeling that the first visa was not issued under AF rules and therefore is a regular spousal visa. If it had been issued under AF rules, you would not be needing to apply for an extension at all, because the visa would be valid until 5th Nov 2019 and you would just apply for ILR in 2019.
Unfortunately, the visa length should have been questioned when the visa was first issued and sent back to UKVI for correction to the proper 5-year visa.
I think this means that you'll need to use FLR(M) to apply for this visa instead of FLR(AF) because your current visa hasn't been issued as an Armed Forces visa.
Since the documents are the same for both, it should just be a case of printing and filling out FLR(M) instead of FLR(AF).