Question on an unusual Martin "Baltimore" photograph

[Stargazer]

While researching some Martin aircraft, I came across this most unusual picture.

Why unusual?

1°) It depicts a Martin Model 187 in U.S. marking. The "Baltimore", as the British called it, was never procured by the U.S. for their own purposes and the planned XA-23 prototype was cancelled before being built.
2°) It sports a most peculiar civil registration: "NXM53." Now I have never seen such a registration before. The NX- prefix no doubt indicates a civilian-owned prototype, but the additional third letter "M" doesn't make sense to me. I guess it might have signified "military", but considering the numerous privately-owned prototypes and demonstrators evaluated by the military in that period which sported the regular "NX-" prefix, this doesn't make much sense to me.

Could this have been the first built example, test-flown by Martin before shipment to Britain? Or an example borrowed by the USAAF for evaluation before allocated the A-30 export contract?

Any help on this matter will be greatly appreciated!

 

[Jemiba]

Found via Goggle two photos of Baltimores with US markings (http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=264 and
http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=264), as it seems both are shown before their delivery.
Maybe this serial was just a temporary one, indicated by the third letter ?

 

[Antonio]

There is a Martin Model 187-B2 (A-30A, Baltimore IIIA) in US Markings at Les Avions & engins Martin. Guides Larivière.
Acording to the text: "destiné exclusivement aux Britanniques... but few were retained in the US and operated under designation RA-30"

It's the same photo as shown in Jemiba's link but in the book it looks a bit better. The number in the side seems to be "FA105". I'm not 100% sure.

 

[Apophenia]

NASA CRGIS has two views of that same aircraft in its Langley Research Center collection. The caption reads"

"The Martin RA-30 Baltimore was a light bomber ordered by the Royal Air Force. Some examples were retained in the United States as part of a "Reverse Lend-Lease." This example was flown by the NACA from June 1943 until March 1944."

http://crgis.ndc.nasa.gov/historic/RA-30_Baltimore

 

[Stargazer]

Thanks. It's very interesting... but doesn't really help solve the above mystery!

I realize I forgot to attach the photo in the first post, so now it's there. As you can see, there are no military markings or camo whatsoever...