I prefer the long handle L wrench with thestar shaped cavity. It seems the long handle gives more leverage and allows finer tuning.
I own a couple of A model Appalachians from about 1967 (according to Becky's Autohrp Book) and I've modified them to two key semi-diatonics.
The D/G harp has a C and a C#, making a D7 chord available in the key of D. I also have a C/G with F and F#, giving me a G7 in the key of G. I also have a couple of old black boxes and a Chromaharp in single keys.
I've never found it necessary to depress two keys in order to play single notes as GUEST,Carol's Friend Don suggested. The felts damp the strings on either side of the string you're after and that seems to be enough for me.