Suggestion:
I recently needed to repaint the lids of a set of West Bend canisters from the 1950s, and a special need was involved: 1950s paint did not have the urethanes and other gloss enhancers that modern paint does.
The answer was Rustoleum Painter's Touch Semi-Gloss spray paint. It was a very, very decent match to the one canister lid that was undamaged. I sprayed until wet, allowed to dry for fifteen minutes, then sprayed again until wet. I ended up with lids that looked 1957 factory-fresh, not over-glossy and repainted.
You are not likely to be able to get this product to match your AMP by spraying, but what you could do is to spray different colors into a container to mix them up to match your AMP. You might try white and ivory to start with, maybe a little black if there's some grey in the white as well.
Careful brushwork and then rubbing out should give a pretty good result - perhaps some sanding with a very, very fine sandpaper used wet. But the semi-gloss finish will make your touch-up easier, I think. I'm familiar with one AMP here where I live, and it's that same 1950s look to the paint - not nearly as glossy as modern paints.
Hope this helps, or at least sparks an idea....