If it's the Gordon & MacPhail bottling, then this is a beautiful malt, and you can buy it with confidence. As you say, it is very competitively priced for a malt of that age.
(Incidentally, the G&M association is not obvious....if you quickly glanced at the label, you could be forgiven for thinking it was an OB).
Longmorn seems to be a malt that ages well. A few months ago I had a 36yo Longmorn (SMWS bottling) that was up there with some of the best whiskies I've ever tasted.