The particular piece in the video below, which was forwarded to me by Jean Wakefield, who also made previous suggestions of ladies to be included in the Contralto Corner, is called "In Memoria Aeterna". Richard Vendome - Artistic Director of the Vivaldi's Women project, which recreated the choir of the Pietà with women singing all parts from Soprano to Bass - suggested that the lowest of the three contralto parts, which had a much lower tessitura than the other two, was "possibly written for Ambrosina", a contralto with "a voice like thunder, just like a tenor".
In the video below, the lowest part is performed by Welsh contralto Hilary Summers with the other parts being take by contraltos Nathalie Stutzmann and Alexandra Gibson. The piece is in the key of C minor, with Summers' first entry coming on a solid G3. She stays below the stave for the majority of the work, and hits both F#3 and F3 natural! In fact, as they are at "Baroque pitch", Summers is starting on a F3 and hits an E3!. Excellent stuff.