I agree with Dave, no matter what it is you won't be able fix it yourself. Sounds like a portion (or all) of the backsping has broken off. "Small end chisel tool"? Do you mean one of the flat screwdrivers?
Here's a photo:To me, the missing part seems as if it would be plate-like and function as the lock back. The tool bit it affects is the centrally located and smallest of the 3 chisel/standard screwdrivers, which to me made it a small-end chisel. I've also used it as a small standard screwdriver, though.Regardless, it looks like I'll have to send it back in.
Basically, you want to take the springs from one side and switch them with the springs on the other side. I'll write it up tonight and post it. It's not that hard to do...I don't have time to do it now. gotta get back to the shop.