It's to do with the baffles in the fuel tank. They recommend that you flick roll them with less than half tanks - although, it's been a few years since I've flown one.
Shows 78 knots in the AFM for flick [snap] rolls.
POH:
http://samdawsoncfi.com/files/bellancadecathlonpoh.pdf
Some good info here:
8KCAB Bellanca Aircraft Safety Analysis
8KCAB Bellanca Aircraft Rental
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