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Until you have found the problem, always retard the spark lever before turning off the engine. That will reduce backfires dramatically. Push the spark lever all the way to the top of the quadrant, allow the engine to run for two or three seconds at idle, and then turn off the ignition. The late spark inside the combustion chamber will reduce the tendency for remaining unignited gas to fire = no backfire. In fact, for all Model A drivers with the original distributor and manual spark control, it's a good idea to retard the spark before killing the engine anyway. This way, the ignition timing is already retarded in the distributor for the next time you start the engine. No forgetting to retard the spark next time and breaking a Bendix spring upon engine kickback.
Admittedly, retarding the spark to stop backfiring is only a band-aid masking the cause of chronic backfiring. But it's still better than hearing backfiring and possibly blowing out the muffler! Follow all the suggestions offered so far and you'll probably snare the cause of your problem.
Marshall