I secured Mate #4102 at the Street auction. It appears to be an accurate, untouched original. However, there are variations from others in the 300-500 serial numbers. The pivot bearing cap is attached with phillips screws, only ones on it. These have silver paint on them, like the rest of the motor, and have safety wires thru drilled holes. The exhaust relief has a drain with a round head screw, rather than a plug, as the earlier models.
Lastly, the exhaust tube is brass, with four holes drilled in line about 1/3 of the distance from the powerhead.
Due to condition of this motor, these look to be production changes. Any info about motors with corresponding serial numbers?