I think Dieter Braun always wins this one with a win in the 1969 125cc (Ultra Lightweight) TT, it was his first visit.
Phillip McCallen won the Lightweight Manx in 1988 first time there, plus the Newcomers.
Geoff Duke's first TT was 1950. 1st and 2nd, although he had ridden the 1948 Manx (Retired), the 1949 Clubmans (1st), and the 1949 Manx (1st).