Filled with vintage character -[ HMM ]-

Simplex 35mm soundhead replacement part box, ca. 1950s.

 

It's not often I come across a box with metal side corner reinforcements. Even the inside part of the interlocking box is reinforced at all four corners.

 

One last vintage reference is the 60 Cycles reference, which predates the 1960s adoption of hertz (Hz) as a unit of frequency equivalent to one cycle per second.

Done