Original HM323 exhausts don't have the stamping on them like the US versions do. I found that the replacement exhausts which I think were stamped HM374 did have the stamping as they were designed to fit the CB550K which we didn't get in the UK but it seemed to be a requirement of US emission control which is why they had it.
Oh and just looked in my original 1976 UK parts book and the muffler caution mark is listed as being fitted to the UK version, there is none of the in brackets text that your picture shows, says it was fitted to all models K0, K1 and K2.