I am wondering if the number you found on the old one is actually 05680-C and not 05680-0. The reason I say this is because there is a Demco master cylinder marked 05680-C. Which would make the Demco Actuator Replacement Master Cylinder # 5650 the correct replacement part.
The Demco Actuator Replacement Master Cylinder # 5650 fits all Demco drum brake actuators and is likely what you have.
If you feel that you have something else, you can submit some photos and I would be happy to take... view full answer...