Catalog numbers at NPL are unique. Specify will not let you save 2 Collection Object records with the same catalog number. The format is pretty simple, and the catalog number field on the data entry form won't let you save an incorrectly formatted number.
Simply put, the catalog format is 3 alphanumeric digits, 8 numeric digits, then 3 alphanumeric digits again.
Over the decades, NPL has acquired a variety of collections. Many of these have their own catalogs, and for the most part, we have been able to keep those original numbers. This is why you see so many collection acronyms. WSA, P, UT, TX, BEG, OMB are just a few examples.
Rarely, historic numbers must be reassigned to make them work with the Specify number format. This seems to happen mostly with TX numbers, which used a format of 4 numbers, TX, then a suffix. Sometimes the suffix would exceed 3 digits, especially in cases where a letter was added to show different specimens on the same slab. Cases like this are the ONLY example where we are reassigning catalog numbers. In the past, many Plummer type specimens were reassigned BEG numbers and this is still causing issues 60 years later.