"Triumph re-entered the 350cc class in 1957 with the introduction of the Twenty One, the first of a new range of unit-construction twins."
http://www.motorbase.com/auctionlot/by-id/1214499944
Seems unit construction starts from 1957, one more thing to tell that when i open the clutch cove, i see 59 written in a round circle, does it mean its a 1959 production or what.
My papers says 1958 but since there were no computerized motor registration department back in those days and everything written by hand, possibilities are on higher side for a written mistake.
