Paint testing for originality

[Schonyenko2]

I don't know that you can age date the paint. It should be laquer. Most repaints back in the day were either synthetic enamel, or later acrylic enamel. Repair work/blending was usually done in laquer. Door jambs most times didn't get repainted on a complete spray unless it was a color change.

You can wipe some thinner on a rag across an area of that jamb and see if the paint comes off. If it does, it's most likley laquer, and would point to being original,

FWIW