Is this ad suggesting that one Ballantine's constituent whiskies is Loch Lomond? Correct me if I'm wrong, but I was under the impression that the only whisky distillery that sourced its water from Loch Lomond was Loch Lomond (beloved by fictional characters...).
To the best of my knowledge, you are correct sir. According to distillery profiles from the "Malt Madness" website (http://www.maltmadness.com/whisky/index.html) , Loch Lomond is the only distillery that sources water from Loch Lomond. The only other distillery near Lock Lomond is Glengoyne, and they source their water from a stream in Campsie Hills. Therefore your assumption seems to be correct, that Loch Lomond is used in the Ballantine's blend (or at least it was at the time this ad was created).