The only information I know of, regarding the replaceable end caps is given in Roy Bacons "Norton Twin Restoration" book.
And I quote from chapter 5, "Carburettor and Exhaust", p92, quote: "....Of the five [silencer] types listed for 1974, only one ran on for 1975 with the end cap available as a spare although it had to be riveted into place."
Maybe a call/email to Andover Norton would answer this question?