Hi David,
Dave B likely nailed the problem. If you remove the tumbler and measure the distances from the center of the spindle on the bridle side of the tumbler to the lips of the half cock and full cock notches, you probably will find they are equal or the half cock notch is greater. The photos below show a lock tumbler almost guaranteed to catch at half cock if the trigger pull is too light.
Perhaps it is best just to let the gun be however, some owner may eventually try to shoot it and may damage the halfcock notch or sear in the process. One solution is opening the bend of the sear spring to create a heavier trigger pull. The other solution is to stone the lip of the half cock notch down until it is equal or lower than the full cock notch. That way the sear will pass over it with a lighter trigger pull. If in the past someone stoned the lip of the full cock notch lower to create a lighter pull, your only option to fix the lock may be to stone the half cock notch. Of course you could install a fly detent but that would be prety radical.
dave