Many times I've seen washers with a little hook
to be used in rear triangle dropout tracks.
Those (the washers) are pretty big.

E.g. the Shimano Nexus 3 has them, often in
some bright color like yellow. They are between
the dome nut and the rear fork tracks.

What is the purpose of the little hook or
protruding part, and how is it different from
a regular washer with a rough pattern, pointing
into the frame?

We assume both are pulled 28Nm.

--
underground experts united
http://user.it.uu.se/~embe8573