This looks like Crouzet gearbox.. ( I was using one -the same housing- a long tome ago... it was from washing machine timer, plastic gears)
http://www.beikimco.com/press/pr_ovo...xes_071410.php
Very hard to find these days. And expensive..
The whole thing looks like home made to me?
My suggestion then would be to open it and have a look inside..
Most likely you have broken gears...
There are places where you may find suitable gears, like SPI.
The other way may be to determine the reduction ratio of the gearbox.. and then see what can be done next in terms of replacing the whole thing with assembly of similar characteristics.