3.3.2.6 <unk-frame>

Frames about which scribbu does not know may be encoded as <unk-frame> instances:

(define-class <unk-frame> (<id3v2-frame>)
  (id-text #:init-value ""     #:accessor frameid #:init-keyword #:frameid)
  (data    #:init-value #vu8() #:accessor data    #:init-keyword #:data))

The data field will contain everything beyond the ID3v2 header; i.e. the frame identifier & flags will have been parsed out.