From Barnet site Clutch parts for Triumph - ALL The Bonnevilles and America are right near the top of the page, about 3-4 down.

MT-5-4 is correct part number. I have always just ordered the MT-5-5 from my dealer since they cannot seem to locate the MT5-4, the difference is that you get 5 springs and only need 4 of them. After several big bores I now have a spare set of 4.

A word to the wise is not necessary. It is the stupid ones who need the advice. Pat