After looking on another thread at something I saw a link for a PDF with all the records of Boosey & Hawkes instruments. Having a "Round Stamp" Euphonium and guessing it was older than me I had a search. From what I can make out, my 663xxx Euph was ordered / manufactured around 1980/1981. 4 years before my birth. Cool!
That's the link for any other intrerested people.