Here goes. From three different sources. No one seems to know for sure why they went with the gray P40s but they did. Looks like P40Ms and Ns, so it's cheating a bit to do it with an E, but since it's all AH has, what the heck right?

First image is of "Geronimo". What it shows is the overall gray camo indentical top and bottom and on the spinner as well. It also shows the blue star and bar on the fuselage and black serial number on the tail. I find no indication of individual aircraft letters anywhere, just names on the nose.

Another view of another 45th bird showing the same thing. Black serial, overall gray, squadron badge on the nose.

Best view I could find to support the lack of a squadron number for each aircraft. Note that on this one, no stenciling is apparent.

Since you are a detail person

This one shows that in some cases they masked over the stenciling on the OD camo and removed the mask after painting the gray so the info showed through. It leaves it as an option anyway.

Line up of 45th P40s. Caption IDs them as Ms and Ns.

Lastly a few nose art photos. Note that Geronimo has a drop tank and bombs under each wing. An interesting loadout for a P40

The photo of the CO's aircraft from the other thread.

Link to the squadron badge
http://www.web-birds.com/7th/15/45.jpgHope it helps. The gray reminds me of Light Sea Gray that I used on models a lot.
Dan/Slack