I still occasionally mouont one of them in an aircraft.

The more common use of a ply tray is not far removed from the plastic servo trays... No appreciable weight savings either way. (maybe 1/4 oz... how you apply glue putting the ply one in can make that much difference.)

The origin of the tray is... In some early RC systems the cheap servos were $50 each. At that time... average income was 1/4 what it is now. Being able to quickly and easilly remove the servos from one aircraft (4 screws for the tray... and 3 for the servo arms) and then plop them into another model was almost as common as using a computer TX with multi-model memory is now.