Is there an equivalent of cash transportation in air cargo? Some cargo (cash for banks, precious art pieces, gold, jewellery,...) are transported in bullet proof trucks with armed guards while travelling by road. Sometimes it may also be escorted by police.

Is there an equivalent for air transport? How is protecting handle for such cargo when transported by air (both at the airport (load breaking and ground operations) and in flight)?
Decades ago, when we moved checks at night for Federal Reserve clearing, we would sometimes have sealed bags which were cash.  On my flights, I never had an armed guard on board, but on taking off and landing, we were met on the ramp by an armored truck with armed guards on a routine basis.  

The impression I had is that checks and other paper were the main cargo, and that cash and bullion were minor incidentals.  

Related, items of national security are routinely carried by scheduled airliners, and are placed into the hold by cleared individuals, and then the hold is opened upon arrival, in the presence of cleared individuals who retrieve the packages.  Contents are logged, in the event of compromise.  Similar parcels are also handled by the major parcel and courier enterprises, with slightly different processes.