The serial number identifies it as a 23-A - 45,000 to 127,000. The magazines with the knurled knob are correct. These are designed so the rear rib snaps into a slot in the fixed rear magazine guide and is held there by a flat spring pushing on the front of the magazine - the magazine has to have the bottom pulled forward, using the knurled knob, to bring the rear rib out of the slot in the rear guide so it can be removed - the magazine well is longer than the magazine to allow this forward movement to remove & insert it.
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