However, he's now a bit stymied. He's been doing some googling for data, and has found several comments in various web forums which say that this item has an 'unorthodox' pinout - i.e. although it's a TO-106 package, the pinout isn't what you'd expect it to be. I managed to get hold of a datasheet for it, but that was no help either!
I've already told him that he could substitute it with one of any number of standard NPN small-signal devices and the unit would still work, but he's determined to use the 2N5133 if he can.
If anyone has any info about this reputedly 'odd' pinout issue, I'd be delighted to hear!