We don't use a specific form for a partial release. It's all in how you word it. After stating the recording information of the original mortgage in the spaces provided on your release form, just state something like "That portion of (the original legal) more particularly described as (the description of the portion you're releasing.)
Unless someone like you cares a whole awful lot, nothing is going to get better. It's not.
--Dr. Seuss, The Lorax