013 Subfield $c - Type of number:

Subfield $c contains a designation of the type of patent document identifier in field 013. The identifier is usually a number. Subfield $c may contain a term or a code identifying the type. The source of the code is Standard Code for Identification of Different Kinds of Patent Documents (WIPO ST.16) that is maintained by the World Intellectual Property Organization. The code is a one- or two-character code. Plain language terms are recorded as they appear on the item. Field 013 is repeated if there is more than one number to be recorded for an item.

Examples:

013 ##$a91-13005$bxxu$cpatent$d19920217

013 ##$a67-SC41534$bgw$cC1$d19671108