The 755A was replaced with the plastic 799 talking station. The mechanisms should be the same. The 799 was then replaced with the plastic 23786 Talking Station. I do not own one to examine but am not aware of a mechanism redesign. These are not covered in the Factory Service Manual.
PortLines has all the repair parts listed on their site. Some are under "station" (bushing, fiber washer & reproducer tube), the nut is under "nuts", no size given.