mapi: format pr_search_key

does anyone know format mapi ability pr_search_key?

the contend it:

the hunt pivotal shaped by
concatenating castle form (in
uppercase characters), colon
impression ':', e-mail castle in
certified form, terminating
zero character.

and sell request says this:

the pidtagsearchkey ability type
ptypbinary binary value shaped by
concatenating ascii twine "ex: "
followed dn object
converted top case, followed
0 byte value.

however mapi messages i've seen ability have arrange binary 16 byte method looks guid. does anyone else have any some-more information it? always 16 bytes?

thanks!