This paper updates Farasyn (2017), who charted the agreement patterns found in Middle Low German (non-restrictive) relative clauses with a first or second person head. In related West Germanic languages, these clauses show different types of agreement patterns. This study presents new corpus data illustrating how Middle Low German compares to these. Expanding on Farasyn (2017), it investigates the historical development and elements introducing the relative clause, showing that φ-features in older stages of the West Germanic languages are always encoded in the non-restrictive relative clause, though often in different positions. Middle Low German displays a remarkable stability in retaining a covert resumptive pronoun bearing φ-features. An apparent competing pattern with an overt resumptive is a later innovation, as are structures with third person agreement found in neighboring languages.