Marcus Karlsson wrote:11 Field Postal Unit which also is part of the Signals Formation.
By way of a historical note : this is the post-1979 version of the arms. The earlier version was tierced per fess Gules [Army], Bleu Céleste [Air Force] and Azure [Navy], with the embowed lozenge in the centre. After the SA Medical Service was raised to equal status with the three older services in 1979, the arms were changed to 'per saltire' to accommodate a Murrey segment for the SAMS. The embowed lozenge was probably derived from the logo used by the Post Office in those days.