Before all the terrible press-fit BB concepts came along, there were three common BB sizes - English threaded 68mm, English threaded 73mm, and Italian.

The threads on 68mm and 73mm are the same, but you generally need different cups for each size because the BB willnot be centred or properly fastened in place if you use the wrong one. Threaded BBs come in both these sizes to match whatever your frame wants.

Italian threaded BB shells are 70mm wide, and the threads are completely different - in size and even in handedness on the drive side (English is LH thread on the drive side and RH thred on the ND side, Italian is RH thread on both sides.)

However, I don't think any of this matters. I think the BB73 the OP is referring to is just a model name for a particular bottom bracket, as BC73 is in the above link. If you have a 68mm wide threaded BB shell on a modern-ish bike (after ~1980), the bike will work with standard Shimano 68mm bottom brackets. You only need to match the spindle length to get the right one.