"Those locknuts are a very commonly lost/damaged part, and have not been
available for decades.

As it happens, there is a relatively easy workaround. Many pedal
reflectors use 4 mm threaded hardware, with nuts that use a 7 mm wrench.

The 4 mm thread is the same as the Barcon pivolt bolt, and the 7 mm size
is small enough to let you use a nutdriver or socket wrench to tighten
the nut up inside the recess in the Barcon housing.

Most bike shops will have a box full of discarded pedal reflectors under
a counter somewhere..."