Classic/Antique Car Repair: engine vibration, powerglide transmission, engine vibration

Question
QUESTION: I recently purchased a 1950 Bel-Air with the 235 C.I.and powerglide transmission. the engine idles smooth but at about 1500 RPM the engine vibrates bad and above 1500Rpm you don't feel it as much. The motor mounts are good. I've been told to look at the harmonic balancer and the flex plate.The harmonic balance looks like something is missing. Coming from the center of the harmonic balancer is this protrusion that is split in an odd shape. I would appreciate any suggestions or advice.
Thanks
Pete

ANSWER: That strange looking part is the head of the bolt that holds the balancer on. It looks strange because it is designed to engage a hand crank for starting the engine. Now I have a question for you. Does the vibration happen when you are standing still in neutral or when you are driving?

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QUESTION: Brad,thank you.I never would have though of the hand crank.I guess GM wanted to use up there old supply of parts. The car does vibrate when parked in neutral and park.

Answer
Now comes the Sherlock Holmes 007 work. Is it a vibration or the engine missing? You can check that by having some one hold the speed up and hold your hand near the end of the tail pipe and see if you can feel a flutter in the exhaust stream. Let me know.