Accessory # Starting With T Vs. A

Hi All,

Why do some Triumph accessory numbers start with a 'T' and some with an 'A'? What's the difference?

Was looking at a seat on fleabay which had a part number Txxxxxx. Couldn't find much on the internet about it but a couple of sites do quote this number.

It doesn't seem to be region specific nor the year of manufacture because the seat in question is for Bonnevilles 2016+.

Any idea?