47301. However, some caution is required on this point for the following reasons: Where there are more than 100 examples in a subclass the identifier becomes obscured, e.g. 31201 is in subclass 31/1, which runs from 31101 to 31327. Some classes renumbered from the 1957 arrangements simply had consecutive numbers from xx001, which ignored the subclasses identified, e.g. 25033 was in subclass 25/1. Where numbers within a class became congested, it wasn't always possible to make the connection, e.g. locomotives in subclass 47/2 were fitted with Green Circle multiple-working equipment but retained their original numbers. In other cases it wasn't possible to start from xxxy01, e.g. subclass 08/9 started from 08991 because subclass 08/0 ran from 08001 to 08958. Because of the different arrangements for numbering multiple units (see above), subclasses may have no link to the unique identifier at all; this is especially true on the Southern Region. Sometimes, where there is a principal run of units and a small variant subclass, the majority will be subclass xxx/0 whatever their identifier, but the variant's subclass does reflect the identifier, e.g. Subclass 302/0 for the standard passenger units (302201-312) and 302/9 for the postal units (302990-302993). So far as renumbering from the 1957 arrangements was concerned, most locomotives retained the last two digits of their number, though some classes were renumbered without reference to their previous numbers. This was usually where sub-classes had already been or were in the process of being created (for example classes 45 and 47, in which some members were being fitted with electric train heating equipment, and 86, subject to various modifications, mainly to the suspension). The other exception was if the first locomotive of a class had carried a number ending in "00" under the 1957 arrangements, because TOPS could not handle numbers ending in "00". These were either renumbered at the end of the class (e.g. the prototype class 50 D400 became 50050), or took a vacant number of another class member that had already been withdrawn (e.g. prototype class 24 D5000 became 24005, D5005 having been withdrawn in 1969), or were renumbered at the end of a sub-group within the class. Class 20 was particularly complicated in this respect. The class had been built in batches between 1957 and 1967 and all 228 members were still in service when the TOPS renumbering scheme was drawn up, but instead of class prototype D8000