" A modification that was embodied on Tornado seats only. Why?"
ISTR this was part of a package of several seat mods (known in some quarters as the "six pack") which were intended to address the problem of Tornado ejected mass growth over time. Part of the mass growth was due to hardware growth (including AEA) and part was due to aircrew mass growth – quite a few exceeded the 99%ile assumed during system design. The latter was partly managed by dietary means but not in all cases; some pilots were reassigned. A more modern 'chute was included to improve seat performance and the strops were strengthened. In the interim while the mods were cleared and embodied, a series of a/c speed limits were introduced to limit strop loads.
Its a long time ago so I couldn't guarantee all of the details.