The early pushbutton motors sat lower and closer to the frame. So much so that if you wanted to remove the header bolt on the #4 cylinder, there was no room to get to it unless you either jack up the motor or do it with the engine out. I've seen them with a slight relief on the frame to make future service to the passenger side header easier.
If it's not a jacking plate, it may well be a spacer to raise the motor a little so there is no need to notch the frame.
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