That's correct after 71 they went to the snap on speed odometer, you just have to push a little harder to make it snap on. I hope you put some lube of some kind on before reinstalling it. They make a lube just for speed odometers. Just thinking out loud.
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Andy
71's Are for ever!!! or 67,68,69,70,72
http://geocities.com/chevy71tpi/main.html