William Bendix in “Lifeboat” (1944) 20th Century Fox.

This is one of the iconic Hitchcock cameos mostly because of
the logistics of it. After all, this is a film about a group of people
stranded on a lifeboat. How could Hitch finagle himself in? Why
by having himself appear in an ad in a newspaper. Hitchcock
can be seen in the before-and-after photos for Reduco Obesity
Slayer.