Seat should be the same, Triumph has it broken down by VIN 55071 and below and 55072 and above, but lists the same 4 seats for both VIN ranges. That is to say, there are 4 different seats for the Trident and all 4 fit both the older (square taillight) and newer (round taillight) bodywork.
It is a simple question for people who can look at more than one parts fiche and compare numbers, but that seems to be too much to ask from some of these parts people...
Cheers,
-Kit