Is there a VIN plate on a T120? I'm not sure. Engine number on top of lh crankcase, just below the barrels. Can't remember exactly where the frame number is, but may be on the headstock in front of the petrol tank. Hopefully it matches the engine number. 'Tis over 40 years since had a T120 to play with, so this info may not be as accurate as I would hope! Value depends on year/condition/quality of restoration (if any) etc etc, but would generally be in the £1,500 - £10,000+ region. Does it run, is it a basket case, is it a show stopper? Ultimately it's worth what someone else will pay for it!
try and find those engine/frame numbers and we'll begin to give you a year of manufacture and model, you should be able to tell if it is pre- or unit by wether the right hand side crank case/gearbox is 2 or 1 piece. also throw in a photo or 2 if you can, specifically of that area and carbs. front wheel.
I take it you do not have the V5 registration document? Put a picture and above information on here. Tap Triumph T120 in google search and then images. see if that helps identify anything. Short answer is frame and engine number are needed.
Here is a pre unit T120 .....wish it was mine https://upload.wikimedia.org/wikipedia/commons/6/62/ZweiRadMuseumNSU_Triumph_Bonneville.JPG and a selection of 70's unit models https://www.google.co.uk/search?q=1...&ei=o4-RVYDaHsfpUuvigMAF&ved=0CCsQ7Ak#imgrc=_ If it is value you are interested in, the pre unit models are now commanding higher prices
