According to the eZ Find 2 documentation it should be possible to user class_id:id as filter.

fetch( ezfind, search,
hash( query, 'eZ Systems',
filter, 'class_id:1' )

But this does currently not work.

If you instead use below it works fine.

fetch( ezfind, search,
hash( query, 'eZ Systems',
filter, 'meta_contentclass_id_si:1' )

So maybe its a type in the documentation?

Steps to reproduce

Install ez Find 2, run a simple fetch and use class_id as filter.