What GZip extra field subfields exist?

Question

RFC 1952 (GZIP File Format Specification) section 2.3.1.1 reads:

2.3.1.1. Extra field

   If the FLG.FEXTRA bit is set, an "extra field" is present in
   the header, with total length XLEN bytes.  It consists of a
   series of subfields, each of the form:

      +---+---+---+---+==================================+
      |SI1|SI2|  LEN  |... LEN bytes of subfield data ...|
      +---+---+---+---+==================================+

   SI1 and SI2 provide a subfield ID, typically two ASCII letters
   with some mnemonic value.  Jean-Loup Gailly
   <email@hidden> is maintaining a registry of subfield
   IDs; please send him any subfield ID you wish to use.  Subfield
   IDs with SI2 = 0 are reserved for future use.  The following
   IDs are currently defined:

      SI1         SI2         Data
      ----------  ----------  ----
      0x41 ('A')  0x70 ('P')  Apollo file type information

   LEN gives the length of the subfield data, excluding the 4
   initial bytes.

Do any subfield types exist beyond the AP given in the RFC? A web search doesn't find a list; neither is there any mention on GZip's Wikipedia page, the GNU homepage, in the gzip source code, or on Stack Overflow.

Answer 1

As far as I know, there is no such registry being maintained. Jean-loup no longer works on gzip.