What was used as paint on insulation/conformal coating in the 1950s?
Photographs are not reliable color indicators.  And there are greases near that color.
But that really looks like the signature color of GE’s Glyptal, which is an insulating paint specifically made for electrical equipment.   Glyptal comes in very basic colors like black and white,  but that particular burgundy is their signature.
Given the 1950s era, Glyptal makes a world of sense, since the product was in its heyday then.
I’ve never had occasion to stick a soldering iron on Glyptal, so I don’t know what it does.  But it does not like to burn, unlike normal paint.
Glyptal is readily available today, and is used for such purposes.   Also, auto racers paint the inside of engine blocks to capture any remaining casting sand, so it doesn’t wind up in the lubricating oil.  I suspect they are after the “non-flammable” characteristic, not its dielectric strength.