There were very few classes with large gaps in 1973.
Class 03 followed the usual pattern of preserving the last two digits (three in the cases of classes 03, 20, 24 and 55), with only the class leader given an out of sequence number - not needed in the case of class 03 as D2000 had already been withdrawn.
Other classes, especially those where the class leader's number did not end in 00 or 01, were generally numbered in sequence from 001 (eg classes 01, 31/0, 46), with any gaps closed up (eg Class 09, 31/1, 47/0, 81). This pattern was also used for Class 08, in order to fit them all in
Classes with more-or-less random subclasses also had the gaps closed up, with newly converted examples added on the end as they were converted,, eg classes 31/4, 45/1, 47/4, 86/2 (which of course left gaps in the subclass from which they had been converted)