I have seen those on a Winchester 101 XTR Featherweight (made in Japan) 12 ga. and they are exactly what the OP thought. They prevent the gun from firing when it is upside down. they seem to work on gravity alone and when you flip the gun over and release the sears, the hammers just catch on the hook. If the gun is still apart, give it a try and you will see exactly what I mean. Quite clever and simple but I'm not totally sure about the rationale.