I've seen those Pan ones, which have a number on the front. That's just one run of books among thousands, and I think they are probably the only ones. But we're being told, "almost all paperback publishers from penguin books onwards allocated their own, unique within the publishing house, cat#s to the titles they published".

So, if I pick up an old paperback from my shelf, chances are it will have a cat number? I've tried it, and they don't. Someone show me an old Penguin paperback with a catalogue number, because I just don't see it.