Hi MattKas,
( EDITED 6th May - Changed Photos and additional wiring info ) :
Do you know if there IS a particular Triumph Model that uses the type of lamp & holder that you're looking for ?
I have looked but can't find one, so there may not be a "Triumph" Part number

There are similar "universal" 1157 bulbholders in the UK, but the locking lug positions are different
to the ones used on the Trophy, which have 3 equally spaced lugs - like this :


There is also the issue of electrical connections - so you'll need a new "wired" bulbholder.
The closest I can find are bulbholders used on the Land Rover Freelander Mark 1, Rear Bumper Stop / Tail lights ....
Here's a link to someone in the UK that sells these -
Bulb Holder Freelander 1 Bumper Stop/Tail lamp/lightMaybe if you contacted them, they could send a Photo showing the locating lugs in more detail ?
If it looks promising, you could visit your local Land Rover dealer to see if they can compare the fitting ?
Then, if the bulbholder actually fits into the housing, all you need to do is modify the wiring by splicing
the wires from the new fitting, into the exisiting wires, and leaving the Triumph moulded plug unconnected

Good luck - and let us know how you get on !

