Almost certainly a 68mm bottom bracket width as that size was in use for ages before 73mm came along....but I'm sure someone else will correct me if not. Probably would have been a closed cup bb originally but you could probably fit a later type provided width, axle length and taper are suitable for your crankset.
I'm not sure what cranks you have, if they're original and you want to keep them then you might be looking for a 122mm axle with square taper or a 113mm with square taper. Can't rememember when 113mm axles started to be used (they were later than 122mm). Both axle lengths are still available in shimano un54 flavour.....see http://www.chainreactioncycles.com/Mode ... elID=17473