This is a regular Kludge Audio Model 506 with differentially balanced inputs and outputs for a 500 series rack. I purchased a matched pair of them in 2021 and used them for a couple of projects, so it has only been used sporadically and is in excellent condition. The modules' serial numbers are #111 and #112 (pictured in dual photo montage). Choose the option of two units if you want a matched pair.

Kludge Audio is a small U.S. based company and currently sell these new for much more brand new—$799—on the music equipment marketplace Reverb. Here is their marketing blurb:

"The Kludge Audio 506 equalizer is the inductor-based equalizer that everybody wanted in 1975. But we couldn't make it then. Nobody could. The technology didn't exist.

The Kludge Audio 506 equalizer is a standard inductor-based topology just like we were using in 1975, but it's built with materials and performance that were impossible back then.

We started with a classic inductor design and took it to the limit with custom built inductors and precision amplification that sound cleaner and more subtle than anything possible before.

It's the best of old and new. It's an old design, built by hand with traditional methods and solid design, but built to be cleaner and more accurate without losing the characteristic sound of the inductor equalizer.

No fancy options. Just three bands of great sound, with tunable top and bottom shelf filters and a widely-sweepable midrange peaking filter. Not too wide. Not too narrow. It's just right."

[www-dot-reverb-dot-com/item/6948051-kludge-model-506-equalizer]