I bought this HP 59313A on eBay, not to use but as a really nice, yet reasonably priced, project case for a piece of test equipment I'm building. These units are fairly obsolete, it's a four channel ADC with a max sample rate of 200Hz. I'm not sure what the exact applications of these were back in their day, I think these were used as part of one of HP's automated spectrum analyzers. This one is an option 001 model, 'hospital grey faceplate', so there might have been some sort of medical use (a common one too, option 001 units aren't rare).
I opened it up an found it was a classic eBay purchase. There should be three large horizontal PCBs in this unit, the top and middle PCBs are missing as well as the riser board that connects them. Oh well, I just wanted the case.
It's a classic unit of this era from HP, the build quality is really high. Almost every IC inside has a proprietary HP part number, but I don't think any of it is custom, just customized labeling of off the shelf parts. The bottom PCB has the input conditioning and switching, dual slope ADC and power supply. The missing PCBs has the control and HPIB logic, and aren't that interesting anyway.
Interesting features of this unit are that the whole PCB is conformal spray coated apart from small masked off areas around the connectors and the epoxy dipped power transformer. And here's what I was after, the stripped and cleaned case ready for a second life: