The part is one of two that fit into slots on the window and prevents the lower section of the window from being raised too high (child-safety reasons). The part is two inches long and 3/4 inch wide (not counting the spring-loaded triangular part that "pops out").
Please let me know if you have anything like this. I appreciate any help you can provide.
![vent lock](/images/pictures/2014/10/42353.jpg)