@oristarA,
oristarA wrote:PS. I'm not clear how "the undersigned" can modify "a committee" in "The undersigned a committee of the Senate"
"The undersigned" doesn't really "modify" "a committee" - rather, "a committee" explains or states who "the undersigned" are.
"The undersigned" is a phrase often found in formal or legal documents especially contracts which means "the person or persons who signed this document".
The undersigned (the persons who signed this document) (who are) a committee of the senate...
Probably nowadays we would insert a comma after "undersigned".