Yes I have seen and handled a number of them, also made a couple with Rose Wood handles for friends to complete a set of tools for a cased Flintlock pistol. If you google Flintlock pistols and take a look at images you will se a number of Turnscrews including ones with a single side blade also. On most guns fitted with forend sliding pin fitting you will see that the side with the head has a small downward facing extension. This making a space between the forend escutcheon and the slide pin head to place a turnscrew blade rather than what people sometimes think their finger nail to pull out the pin to release the forend.

Ernie

The only lessons in my life I truly did learn from where the ones I paid for!