It's often asserted on this and other forums that the letter(s) at the end of a (9 character) part number is 'just a revision level'. People claim and assume that an alphabetically later letter or pair of letters is the same part but new and improved, or from a different manufacturer, or whatever. They assume it'll fit and work   But it simply isn't true, not all the time by any means.   Example, brake discs with core part number 6R0 615 301   Without any lette