Those where used in industrial control equipment. Some of the have very good DC characteristics. Some are low noise.
I even had one Burr-Brown that was doing modulation/demodulation of the DC in order to amplify it in the AC domain.
If it's a Philbrick you might contact Bob Pease, who recently retired from National Semiconductor and loves those old units. He used to work at Philbrick.
Google his name. He lives in Northern California I think.