No pressure then, Fishless!
However I think you have it right that this is Ann Robertson's mark. Her workshop seems to have been much more prolific than Kelty's as we see many more instances of AR than of AK so balance of probablities favours her too. I have copied the relevant Ann Robertson mark from Margaret Gill's book on Newcastle silver and put it beside a version which I chopped off in approximately the same place as our original mage:
Phil