Looking for National Insurance Category Letter

Summary

I am looking for the NI Category Letter

Content

Does anyone know where in the tables (OTBI or BIP) I would find the National Insurance Category Letter? 

Just to clarify I am not talking about the Tax Code. The Category (or Table) Letter is one of:

A, B, C ,H ,J ,M ,Z.

Post people are on 'A'. I've searched a few tables but of course it's not easy to find. 

Any ideas of table and field name greatly appreciated. 

Oh, and it's not on 'per_national_identifiers'.