A receiver for the Decca navigation system. The set has three dials on the front, each marked from 0-1 with divisions of 0.1, subdivided to 0.02, and with a knob to the left of each. The dials are marked successivley R, G and B, indicating the different Decca chains to which they corresponded. A panel beneath the dials swings down to reveal controls for adjusting, setting the appropriate chains, etc. The whole set is held in brackets that allow it to be inclined.

Decca Introduced the Mark 21 receiver in 1969. It was the first in which everything was contained in a single box unit, which was suitable for bulkhead, deckhead or table mounting. The Mark 21 proved to be a very successful model, only being replaced by the introduction of the Mark 30 in 1980.