ID replacement for sash operator

Milgard logo on crank handle. Number 31858 stamped on lower arm (not one of the arms on the "dimensions" photo, but shown on "all parts" photo and closeup. Date and time shown on underside of unit -- 08/25/00 10:28. House was built in 1952; so window has been updated since the, but before our arrival in 2018. I hope there is a standard operator to fix my problem, including the 5/8" square plastic part at the end of the long arm that mates with the channel on the bottom of the sash frame.