Seattle Pioneer.........This is a followup from your old post (very old) that was recently resurrected and closed.
You were making mention of a device you made for powering a millivolt system to test it. I purchased a meter around 1967-68-69 that did that very thing.
Honeywell Systems Tester W720B1011. I hadn't even thought about it in 30 years, but I found it in my rollaway. Still working condition except the battery holder is damaged from leaving the batteries in. I don't think I ever used that function.