Thank you all.
The "penny dropped" in the early hours of this morning. My apologise for being so slow.
Both SB's are valid, one for certified , the other for uncertified engines.
If my serial numbers are in one of the documents, it will not appear in the other, however the SB remains valid.
My task now is to find out if this work (replacing the circlip) has been done or not, on my carburettors.