or
"MONITOR BLACK"
Originally released as the "Concentric Dual",
but populary referred to as the "Monitor Black" this design was
conceived around 1946 by Ronald Rackham. This 15" speaker was originally
developed as a reference tool for measuring microphones, but was soon launched
as a product in its own right. It was displayed at the 1947 Radio Olympia
exhibition in the UK, and quickly gained credibility with the professional
market (through orders from the GPO and the like).
This design remained as the basis for all Tannoy dual concentrics for about 30 years (up to and including the Monitor HPD of the late 1970's).
15" Monitor Black:
The chassis casting of the 15" Monitor Black differs from the design of the later models. The basket of the 15 " Monitor Black has four straight spokes from the outer ring back to the magnet assembly. The basket was finished in black "crinkle finish" enamel, and has a very substantial appearance.
12" Monitor Black:
The 12" Monitor Black was released after the 15" unit. There are production variataions in the baskets and paint for this model - some early units have pressed-steel baskets, painted all-over in black gloss enamel. Other units have a cast, silver-painted, chassis but retain a black-enamel painted magnet cover.
(See comparison at above-right: Monitor Black 15" [upper] versus Monitor Black 12" [lower] with cast chassis and a black magnet cover).
In 1949 the Tannoy 12" Dual Concentric loudspeaker
was demonstrated to Decca, and they subsequently placed an order for 900
loudspeakers to be used in the Decola gramophone. Later, in 1950, Arthur
Haddy, (Chief Recording Engineer at Decca) heard the Tannoy Dual Concentric,
and ordered them for used at the Decca FFRR Studios. The loudspeakers were
named the Monitor Silver. 
The rapid industry recognition for the Dual Concentric continued when EMI ordered them for the Abbey Road recording studio in 1951.
See also, the lauch of the Dual
Concentric at the London Radio Show, 1947
on the Tannoy web site (click thumbnail image at right).
(Some historical details above are sourced from www.tannoy.com)
Above: Advertised Frequency Response of the 15" Dual Concentric Monitor Black.
(Photo courtesy allofus@es.co.nz)
(Photo courtesy allofus@es.co.nz)