This HO Scale cupola is designed for Canadian National's modernized snowplows. Originally built with a flat front, CN added the sloping front window section to reduce snow buildup while plowing.
This cupola is narrowed to fit the Walthers Russell plow, which some modellers are using as the basis of a kitbashing project to represent the CN plows.
A second version, completely accurate for the actual CN plows, is available at:
https://www.shapeways.com/product/H5GYGL3N5/cnr-plow-cupola-sloping-front-ho?li=shop-results&optionId=57762656
This part is available in FUD and FXD acrylic.