You're buying the FROG for a Stanley #20 compass plane. 

This frog is from an early plane with the patent date of June 17th 1879 (cast into the dovetail on the sole).

And three patent dates on the lateral adjuster, 76,84,88.

The frog is in very good condition, and retains much of the original plating as can be seen in the photos. the only notable defect is that the lateral adjuster was bent and was straightened. 

The frog has been carefully cleaned, lightly oiled and ready to install. 

Contact me with any questions.