Truth Entrygard Dual Arm Operator. We would expect you to find 20755 on the bottom of the linkage arm, which measures 3-3/4" overall length. Other common numbers we would expect to find are 31538, 31539, 30841, 30842, 30472, 30470, and 20755.
To determine hand of operator, if your sash lock (window lock), is on the left, you would use a left hand operator. And if your sash lock is on the right, you would order the right hand operator.
3-3/4" link dimension, offset down. For offset up order Wi1516
Order Wi1510 if your link dimension is 4-7/16" stamped 20810.
Order Wi1515 if your link dimension is 2-7/8" stamped 20748.
Left hand shown.