A Rare First Find

I am a bench mark missionary. A non surveying friend happened to stumble across this one today and was so elated (I think I'm over doing it here) he sent me the photo. It's his first BM find and he's managed to find a rare one.

We have a BM cut mark bolt on Exeter Cathedral but I didn't realise until today that they are not terribly common. Indeed, some geeky BM sites list cut marks with bolts as "highly prized by benchmark hunters". And to prove a point Jochta's bench mark website makes me look like a Mickey Mouse amateur at this game. In my bench mark googles this afternoon (I really should get a life) I came across Benchmarks.org.uk - what a find. It lists all the levelling routes taken. Bizarely I couldn't find this Fareham BM on it but perhaps I was just getting a bit bamboozled by it all by this stage.

Someone once told me that BMs with bolts mean that they are related to more than one datum but I can't find a source for this info. Anyone know?