Have to say that "where the pong goes" sounds rather contrived.
I prefer the version that says in the 19th Cent. pongos - oops! sorry, lobsters wore scarlet tunics, just like the scarlet jackets traditionally worn by organ grinders' monkeys. And as pongo is an old name for an ape...
Betcha that's the real origin.
Interestingly that while the OED lists pongo as slang for a soldier it does not list crab or crabfat in this context which is surprising so crabfat must be another forces-specific word that has had virtually no exposure outside.
And as a nickname "Foo" Kinnell takes some beating. Or the infamous "Piggy-Eyes" Nicholson.
Edit to add this little gem lifted verbatim (that's all of it to pongos) from arsey pedia. (which, you pongos, does NOT mean bum boys...)
Origins of 'Crab'
While some of the explanations below are derived from or purported to be the "official" version the RAF have been named "crabs" due to the fact that they have no limit to the number of sideways paces they can do as a drill movement. While army and navy have a fixed limit of sideways paces the RAF can actually march the entire width of a parade ground sideways.
Once upon a time, the RAF flew Spitfires with tail wheels near the tail and huge Merlin engines at the front. This meant that during taxiing, all they could see to the front was an enormous engine. In order to see what was in front of them, they had to look over the side and zigzag, thus they'd be moving sideways. On landing, they would sideslip to lose height.
Because of airfield security, the only time land-locked mortals got to see the RAF anywhere near close up was during landing, take-off and taxiing, when they were moving sideways. Since only crabs walk sideways, clearly the RAF was staffed by crabs.
Coupled with the fact that the Fleet Air Arm attracted all the best pilots and the Army always won inter-service regattas, a once popular forces T-shirt logo read: 'Fly Navy.Sail Army. Walk Sideways!'
Another more plausible explanation as to why the RN refer to the RAF as 'crabs' goes back to the days of rum, sodomy and the lash (about last week in fact). One of the more unsavoury aspects of the average matelot was his unfortunate habit of contracting pubic lice or "crabs" during his shore leave. The treatment for this condition was to get a chum or shipmate to apply a liberal application of a greasy blue/grey ointment (known affectionately as "crab fat") to the affected area. The proper name for the ointment was Blue Unction.
With the RN's usual powers of wit and sophistication the RAF were thereafter referred to as 'crab fats' (or crabs for short) as their blue/grey uniform was exactly the same colour as the stuff that the filthy little ratings rubbed on their swollen and lice-ravaged goolies. The RAF by contrast affectionately refer to the Royal Navy as Bum Boys or Fish Heads. The Army are Pongos, Brown Jobs or Grunts.