The Contract.signer element should simply be the Signature datatype. As it is modeled today the Contract has Contract.signer.type and Contract.signer.party; which are duplicates of Contract.signer.signature.type, and Contract.signer.signature.who. I suspect these left behind elements are from an earler Signature Datatype. Now that the Signature Datatype is stable, the Contract.signer should just use the Signature datatype alone.

Contract.signature 0..* Signature