This joint issue between Portugal, Brazil and Spain, commemorates the Treaty of Tordesillas, signed in the eponymous Spanish town on 7 June 1494, between Portugal and Castile. The treaty establishes (before the discovery of Brazil!) a line 370 leagues west of Cape Verde, already settled by Portugal: territories west of the line would belong to Spain, those east of the line to Portugal.
The Cape Verde stamps are part of an earlier issue with two further stamps that commemorate the two 1493 bulls of Pope Alexander VI, which gave all territories in India and 100 leagues west of Cape Verde to Castile. Those bulls led Portugal to negotiate directly with Castile. The Tordesillas Treaty counters those bulls and was sanctioned by a papal bull in 1506. The design of the Cape Verde stamps is very clever, showing the location of he Cape Verde archipelago and representing the Tordesillas line by the perforation between the se-tenant stamps.
The Portuguese stamp was designed by Vítor Santos, lithographed by the Mint on enamelled paper sheets of 5×5 stamps with perforation 12×12½, and circulated from 7 June 1994 to 31 March 1999.
The Spanish stamps were lithographed with perforation 13½×14 and issued on 7 June 1994.
The Brazilian block was lithographed on phosphor paper with a stamp of perforation 12×11½, and was issued on 7 June 1994. The undenominated stamp is for international letters up to 20 grams, which cost 1689.02 cruzeiros at the time. This was the last Brazilian issue before the real (1 real = 2750 cruzeiros) was introduced on 1 July 1994.
The Cape Verde stamps were lithographed by Courvoisier on glazed paper with perforation 11¾×11½, and issued on 1 August 1993.
Reference: Mapa-mundi de Bartolomeu Velho (1561), Mapas de Santos, accessed 25 April 2015.