[R-390] Weird looking set screws

[Tisha Hayes]

The R-390/ 390A use set-screws that are designed for a much higher torque
application.

It is known as a "Bristol-Spline" screw head. It may look like a hex or a
little like a TORX but the internals are all right-angles instead of
triangular or hexagonal faces.

The old Xcelite tool-set included Bristol-Spline bits, back in the day most
people had no idea what to do with the Bristol Spline bits.

The reason for Bristol-Spline is that when tightened it will not cam-out
(strip) like a hex bit will.

*Ms. Tisha Hayes, AA4HA*