Triumph Trophy 900 and 1200 wheels

Can't find the exact info for you but in the book Hinckley Triumphs the first generation the specs for the 1991 trophy 900 and 1200 is the same,wheel size 18x 4.5,tyres 160/60x 18V280 so I would assume they will be fine. The bikes were modular and used many common parts, They did go to a 17 inch rear rim later .on tho
Just checked in another book, Triumph motorcycles,their Renaissance and the Hinckley factory. The later Trophy 900 and1200 both used the same size 17 inch rear and the same size tyres so I think this would confirm the sizes you have are ok

 

The owner's handbook for my '95 Trident 900 (yes, I do still have it, though it's starting to show its age a bit - much like me!) shows every bike in the range at that time has a 17" rear wheel, apart from the Tridents, aka 'The Special Ones', coz they have 18" jobbies.
The Haynes manual shows all bikes as having 17" rear wheels, except the Trident (18") and the Thunderbird (16"), but there is no mention of model years.
As per Sprintdave's post, I would say either wheel will be okay due to the modular way of manufacture of the early bikes. Your bike could easily be dating back to the time they changed over.
Have a look at www.worldoftriumph.com in the 'parts' section for your bike to see if they list both sizes and for which model years.
The basic difference between the different models back then was pretty much just the bodywork, although the number of cylinders and the state of tune of the engine were other variables.