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The 21FDP4 should work OK with only a single wire change on the socket. The tube is about a half inch longer.
Drill out he rivet holding the socket cover and move the wire and connector from pin #3 to pin #6. The heater voltage is 6.3 vs 2.35, but the current is the same so nothing needs to be done. If you do it (except for the drilling) while the socket is on the old tube, the other pins will stay in place. Either tube will work when you're done.
Put the socket back together with #2-56 screws/nuts or whatever fits.
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