Interesting....
The IMDB only has 5 more credits for him after The Avengers:
Diagnosis: Murder (1975) (story)
Deadly Strangers (1974) (story)
The Firechasers (1971) (as Philip Levine)
Mister Jerico (1970) (TV) (as Philip Levine)
He was born in 1926 so only 43 when he 'retired' or moved on to other
interests....
Good question!