Part no. 42 is loose. It appears that this is a common problem especially since there's already a shim solution. Thanks everyone.
Right. It’s all nomenclature:
#42, what CZ calls a “trigger unit casing / lower housing” is what we AR-centric Americans would call a lower receiver / trigger housing.
What we would refer to as a “trigger pack” is #47 “trigger case” assembled with all its component fire control group parts.