Ah, but wait! Correct me if I'm wrong (but I'm not ) Does not the 11th Doctor use his Sonic as a flashlight at times? The latest episode for one.
So... reversing that logic (and stretching it somewhat): If the current Sonic can be used as a flashlight, then the fact that the 1st Doctor uses his device as a flashlight does not preclude the possibility of it also being a Sonic.
Thoughts?
I suppose. But he takes the light out and gives it to Ian which he probably wouldn't do if it were a more complicated tool. Also when he's showing it to Susan it's very clearly treated just as a light with one on/off button.
I notice the same (or a very similar) penlight is used in the serial "The Sensorites" and he calls it a torch.
You could twist it and call it an assumed sonic but I think it's conclusive that the first on screen sonic screwdriver was Troughtons.