It depends on the brand. It might stand for "Taurus Handgun" if it's a Taurus. What brand is it?
The number to Taurus customer service is 305-624-1115.
It is a key operated lock on a Taurus handgun. When the lock is ON, the gun cannot be fired. Go to the Taurus website, and you can see how it works.
100-300 USD or so
50-250 usd
50-350 usd
around $30-40
50-225 USD or so
50-200 USD depending on condition.
100-260 or so
You need the services of a gunmsith.
See the Blue Book of Gun Values