Numrich gun parts lists all rear sight parts as being available, except the blade itself.
The model 940 schematic does not show the parts as being available.
But the model 939 schematic says they are available.
It is the same revolver, except the 939 has a slab sided barrel, while the 940 has the round barrel.
Also I never rely solely on the website to check availability.
I always call and talk to customer service, they are very helpful, and many times have had what I needed when the website didn't show them!
ps have your serial # handy!
http://www.gunpartscorp.com/catalog/Products.aspx?catid=7930