Coff: The full list of Chipmunk modifications, including dates of approval, can be found on the DHSL website here:
http://www.dhsupport.com/archives/Ch...ifications.pdf
Also, the glider towing mods were not installed during production. Modification H.197 introduced glider towing, and that was not approved until 4 December 1953, some time after production had ended.
For information, the fatigue improvement modifications are:
H.288 - centre fuselage steel lower tie bar.
H.289 - mainplane lower spar boom steel inserts.