Shimano don't make them any more, so it's used on eBay or similar. They had hollow shafts to save weight, whereas the current Shimano spare (UN300) is nowhere near as light. Incidentally, you need one for a 68mm shell, not 73mm.
This UN55 is about the best of the new ones around (and also discontinued now): https://brilliantbikes.co.uk/compon...bottom-bracket-british-thread-68-1225-mm.html
This UN55 is about the best of the new ones around (and also discontinued now): https://brilliantbikes.co.uk/compon...bottom-bracket-british-thread-68-1225-mm.html