If I'm not too late to help, you should be able to get a Heritage Certificate from the British Motor Museum https://www.britishmotormuseum.co.uk/archive/heritage-certificates to confirm the car's numbers. The terms commission number, chassis number & VIN are almost interchangeable. On Triumphs the phrasing tends to be commission number until the very last cars which used the new-fangled long VIN numbers.
You can then get a reproduction commission plate from one of the Triumph suppliers and get that stamped up by an engineering workshop - or get a set of stamps yourself. I attach an image of an original plate from a GT6 Mk3 and would suggest that this may be the design you would want, but ideally you might be able to ask the question on here once you know the commission number of your car - it may be the one on the carte gris if that fits within the GT6 range - to check it is the correct plate for the age of your car.
To be clear and above board we own Classic Reproductions (classicrepro.co.uk) and sell various plates etc but you will find that Rimmers and many other Triumph suppliers can also provide these plates.
http://triumphspitfire.com/History.html this is one site that gives an overview of the range of commission numbers used on various cars.
I hope this is of some help.
Guy
