Most specimens look like the one below with a blank field above the plectum, but a few including the one above seem to maybe have a tuning key.
Those with the tuning key may be
O = McCabe specimen above
Looking over the Schaefer binders it seems like the die engravers didn’t like the blank space above a short plectum and kept trying to make it longer and on just a couple dies that plectum extension transforms into a lyre key. I don’t ascribe any particular meaning to this variation, just an interesting feature.
I am indebted to D. Levinson, @Ancientlyric, and James Lloyd for helping educate me on tuning keys.