• The Rat Patrol Classified Dossier - wonderful character/cast bios, articles, an interview with Gary Raymond (Moffitt), the highly amusing Episode Guide and more.
• Sun Compass - a fun exploration of real history and the fictional Rat Patrol world, includes an interview with Lawrence Casey (Hitch).

Historical  Links
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Popski's Private Army - the P.P.A. began as a demolition platoon in the L.R.D.G., then were officially formed as No. 1 Demolition Squadron, P.P.A. with a very specific mission of harassment and sabotage in North Africa under Major Peniakoff (nicknamed "Popski").   Their main vehicles were Willys Jeeps loaded down with weapons, including a .50 Browning.  They did often work in concert with the L.R.D.G. 

• Popski's Private Army Preservation Society
• Wikipedia Entry for P.P.A.

Rat Patrol connection -   Probably the main inspiration for the RP.  A copy of Popski's book was given to the actors and excerpts from it were given to prospective writers along with the RP Writer's Guide to establish the style of the Rat Patrol's missions.
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Long Range Desert Group - A small, highly mobile unit of the British Army engaged in daring exploits and reconnaissance far behind the enemy lines, they were the acknowledged masters of the vast inner desert, often used as guides or support by the S.A.S and the P.P.A. 

• Not By Strength, but Guile - the L.R.D.G.
• L.R.D.G. Preservation Society
• Wikipedia Entry for L.R.D.G.

Rat Patrol connection - Besides the obvious similarities, Troy actually refers to the Rat Patrol as a "Long Range Desert Patrol" (the original name of the LRDG) in one episode.
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Special Air Service - The SAS was founded by David Stirling during World War II to conduct raids and sabotage far behind enemy lines in the North African desert. They operated in conjunction with the LRDG.  Stirling (originally of the Scots Guards, then No.8 Commando) looked for recruits with rugged individualism and initiative.

• Special Air Service Regiment
• UK Special Air Service
• Wikipedia Entry for S.A.S.

Rat Patrol connection -   In the RP Writer's Guide there are notes from Gary Raymond (Moffitt) that indicate that Moffitt's character was partly inspired by Stirling.  Those notes also indicate Moffitt as being of the Scots Guards (not the Scots Greys as said in the pilot), then volunteering for a Commando unit that went to North Africa before joining the Rat Patrol just like Stirling.

Further musings about Moffitt's proper unit and background may be found HERE.