North American P-51 Mustang —

Dan Marotta - this is a pair of F-6D Mustangs... and at the time of this photo might have "officially" been redesignated as RF-51D types!!!

Which is to say that both were built for the "tactical photo reconnaissance" role.

In the foreground is USAF s/n 45-11659. Note that "U.S. Air Force" is stenciled upon the "tail"... which dates this photo to 18 Sept. 1947 or later. On the starboard wing, you can see "USAF" marked. (Only "SAF" is visible due to the photo angle.)

Here's what Joe Baugher's excellent information reveals about USAF s/n 45-11659...

"11655/11689 built as F-6D-30-NT".

http://www.joebaugher.com/usaf_serials/1945.html

The aircraft in the background is USAF s/n 44-13029... another F6-D Mustang!

Mr. Baugher tells us this about 44-13029...

"13020/13029 built as F-6D-20-NT".

http://www.joebaugher.com/usaf_serials/1944_1.html

From this website we glean more about the USAAF (when these were requisitioned and manufactured) and USAF (when the photo was captured) North American F-6 Mustang... a "tactical reconnaissance" aircraft...

http://www.historyofwar.org/articles/weapons_F-6_Mustang.html

"In the F-6D and F-6K, based on the bubble canopied P-51D and P-51K, the oblique camera also had to be placed in the rear fuselage, being placed just above the vertical camera."

The clear window ports for those cameras are seen on the rear fuselage of both Mustangs.

The FF-659 on the F-6D in the foreground (and should be FC-029 on the F-6D in the background) were USAF "buzz numbers" as commonly displayed at that time.

http://www.aerofiles.com/buzz-tails.html

In case that F-6 moniker confuses anyone, the "F" prefix formerly designated a "photographic" mission aircraft. "P" was for "pursuit" aircraft. All of that changed in 1948 and "F" was deemed to designate a "fighter" aircraft... and the use of "P" for "pursuit" was no more. USAF reconnaissance aircraft gained an additional letter prefix of "R" preceding the type of a/c it was based upon... e.g.- RF-51, RF-84, RF-101, RF-4, RB-47, RB-57, RC-135, etc.

That FC and FF prefix observed on two same type Mustangs in the same photo (both are F-6D) and the absence of "US Air Force" on the tail of 44-13029 (also with its "FC" buzz code prefix)... with "FF" as painted on 45-11659 which shows "US Air Force" on its tail leads me to believe this photo is circa 1948.

This was certainly a transitional period for "regulation" identification markings as commonly seen on Air Force aircraft in that era. The above photo is evidence of that.

Wiki notes that in 1948, "...surviving F-6s were redesignated as RF-51D/K".

https://en.wikipedia.org/wiki/List_of_United_States_Air_Force_reconnaissance_aircraft

@Gary Schenauer