Steve, I did see Davy doing it several times & it involved elbow & side of the hand on a hard surface- can't remember exactly how, as I said earlier.
Only version I've heard apart from the one by Davy on the 'Festival at Blairgowrie LP on Topic is the audio one I found by Ray Fisher on YouTube. There is a clattering sound, which could be a drum of some sort or Ray tapping her guitar, or did she do it a la Davy Stewart.
As you said you'd seen Ray perform it several times (I never did) I was just curious HOW she did it- or was it someone else?