I looked in my book for the Great War AFM Role. Curious that better than 90% of the AFM's awarded do NOT have any citation. The ones that do are all related to R101 and R34 Airship work. A friend of mine in the UK is very deft at finding things that shuoldn't exisit in the records. He may have some insight. I did a quick check on Spinks sales history - they have not handled the medal. That doesn't mean it hasn't been sold, donated, in a family drawer or scrapped. It would be a very pricey award. Chances are slim you'd find out. But as I said before, more than likely an 'attaboy' for good service. He was an air mechanic, so engine work of some sort.
Send pics of his uniform items. I'm very interested in these and have a small assortment of RNAS stuff myself.