I guess most of you know the the Dire Straits concert at the Nelon Mandela 70th birthday tribute concert in Wembley Stadium, 1988, and have probably – just like me – watched the video of it many times. I at least have never noticed that on this gig Mark has two output jacks on the famous National Style-O, have you?
One of the two, the one with the lower output jack, is definitely the Ashworth pickup (AJ21) he already had during the Brothers in Arms era, just compare the look and position of the cable to the famous CD cover picture. The Ashworth is a passive piezo element pickup.
About the other we have no information yet. We can see that that the cable runs under the string holder, probably then it goes through a hole in the ‘chicken feet’ on the resonator plate into the guitar body, where the resonator itself rests. We can only speculate what it is exactly: I see mainly two possibilities, one is another pickup that is attached to the resonator cone, or a little microphone. Many acoustic guitar pickup systems use a combination of a microphone and a pickup, like the L.R. Baggs that is used in Mark’s Martins.
Well, it does not matter too much for the sound on this concert because it seems it was not in use here; I can see a plug in only one of the two output jacks. But it shows that Mark always tried out new stuff to improve his guitar sound.