This is an HO scale window replacement for the Rivarossi/AHM steam locomotives. It is based on a Central Vermont RR prototype built in 1880. Remove the original windows from the cab and file the openning to fit. The back window is slightly deeper then the front hence the right/left designation, the smallest dimension to the front. Go to
bigstacklittlelocomotives.wordpress.com for more info, and visit
ottgalleries.com and look up "ending the curse of the black rivarossi"