These are bar mounting clamps for circa 1941 Rock-ola Dial-A-Tune bar boxes such as the 1525.
From looking at photos on line I see that they did use two different types of clamps - there are photos at Hildegard Stamann's web site showing a bar box with this type however.

The mounting holes are 1.5 inches apart and appear to be 10-32 thread.

They are rusty and will need to be bead blasted and then painted.