Internet understood.

I don't know if it's my own terminology, (Like as not he'll put me right) but I know this as "Dry Dyking"
(I nearly did a Steve Wright and said "No "G" ). Out of curiosity I did  search for same, and up popped in excess of 10 pages of references to "Dry Stone walling"; one of which included this wee gem:-
"Craftsman Work Key
Where provided, each entry also shows a series of letters describing the location and range of work undertaken, which are as follows"

I'll not list them all, just the pertinent one...

"G - Holds additional certificate in Galloway Dyking."

He was another cameraphobe, but was happy when I said 
"It's the wall I'm after lad I'll just shoot your back."

"Foreigners" wanting more info - look here.
Galloway Dykes found here.

The extra was to be "it" but due to galloping senility I went to The Birds' Bistro by accident instead of the Milk & Egg farm; spotting this en route to where I ought to be.