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Yes, the knob holding the metal flap should unscrew. Doesn't have to unscrew all the way, just far enough to let you swivel the metal flap.
Handy hint, you can get a better grip on the knob for turning it if you wrap the knob with a postal-sized rubber band. If you still can't muster enough strength, then leaving the rubber band in place and gripping the rubber band with a pair of pliers will give you enough leverage, without damaging the knob very much.
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I thought getting old would take longer!
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