Did you try to crack open a fuel line to cut off fuel to the #5 to see if there was any change? (if need be, crack one at a time so you can compare what you hear on the "good" ones, with what you hear on the #5.) If the wrist pin is messed up, a compression check can turn out normal, and lead you to think nothing is wrong. An experienced tech. should be able to listen and tell you more. Try to check the compression on them other two, it may provide some clues.
-Larry

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03/83 300D 184k
07/73 280 160k
-88 other 153k