Update: The filter was mounted in an 8" x 6" electrical box.
I took the cover off and removed the filter unit as shown.
When I went to put the cover back on the box, I found it had a small White Instruments label on it! The 2615-1 is the type or model, the 7128 is the date, and the frequency is "Treble".
Making a little progress....
White Instruments is still around; maybe they would provide some more info.
What I have in my collection is a rack mount unit that has 16 octal tube sockets in it-all in series.
Most of them are used-but the one that aren't have a jumper to bypass that position.
You remove the jumpers and put in the filter that you want.
So the "procedure" in the 'old days was to measure (with whatever tools you had-no computers or phones back then) and determine what freq and width filter you needed.
Then you would order one-wait a week or two. Put it in, figure out if you needed more filters and wait.
Not quite as quick as today.
I got mine when I removed it from a Church that was doing a system upgrade.
A piece of eq history.
And don't forget about the White passive eq units that you could adjust the gain of each of the freq bands.
If you didn't properly terminate the output, the freq would be all over the place and not do at all what you "think" they were doing
If we still lived in the "power transfer world" instead of the "voltage transfer world", there would be TONS more questions about interfacing gear than we have now.
It was NOT "plug and play" by any means.