IMHO the 1st guitar should have been the 61 or the national, the 2 iconic Mark's guitar in term of sound and visual
the SOS demo was recorded on 27/07/1977, given to Charlie Gillet, and later featuring on the CD "honky tonk demos"
The SOS single version was recorded on 20/04/1978
And yes Mark bought his 2 national (style 'O' and tricone) long before Dire Straits, and before the 61, I guess in the first half of the 70s