2006.09.26 17:53 "Re: [Tiff] Status of ISO JBIG and CIELAB JPEG support", by Jean-Yves Le Ridant
I agree with Joris that fax000003507.tif seems to be a well formed ITULAB. ( from my materials and Photoshop ).
The linear rescale to apply to a and b to get "possible" colors meet the spec. of ITULAB.
IMO, ( if I undestand and your main goal, if not only goal, is read/write ITULAB Tiff ), it could far more easier and faster, rather than expect have a libtiff processing LAB files, of process yourself jpeg data stream, using the read/write encoded data interface.
The add to Tiff Specifications for jpeg in tiff, well explain the organisation of data against jpeg markers, this meet of course well jpeglib requests and capacities, and is not far complicated.