I bought Triumph's official bar end mirrors, which according to the engraving, are actually made by Napolean. Are these the type of mirrors you're asking about? If not, then disregard my advice.
But if you are asking about the bar end mirrors, the difference is that your stock handle bars have a metal "plug" welded inside, about one inch in from the opening, and that is where the handle bar weights attached with the long bolt. Therefore, the official Triumph bar end mirrors do not have the protruding mounting attachment that are meant to fit inside most aftermarket handle bars, because if they did, the "plug" wold be in the way. Buying and trying to fit after-market Napolean bar end mirrors (the kind made with the more common attachment meant to fit inside the handle bars) with stock Triumph handle bars would create a mounting problem for you.