The submarine depict is one of the type VIIB submarines, of which 24 boats were built on several German yards and in service during World War II in the German Navy.
Displacement 753 ton surfaced, 857 ton submerged. Dim. 66.50 x 6.20 x 4.74m (draught)
Powered by two diesel engines, 3,200 hp and 2 electric motors, 750 hp, speed 17.9 knots surfaced, 8 knots submerged.
Armament 5 – 21 inch torpedo tubes, 4 bow one aft.. 1 – 8.8 cm deckgun. And 26 mines.
Crew 44 – 48 men.
Info on this type is given by Wikipedia.
The German type VIIA submarine had limited fuel capacity, so 24 units of Type VIIB boats were built between 1936 and 1940 with an additional 33 tons of fuel in external saddle tanks which added another 2,500 nautical miles (4,600 km) of range at 10 knots (19 km/h) surfaced. They were slightly faster than the VIIA, and had two rudders for greater agility. The torpedo armament was improved by moving the aft tube to the inside of the boat. Now an additional aft torpedo could be carried below the deck plating of the aft torpedo room (which also served as the electric motor room) and two watertight compartments under the upper deck could hold two additional torpedoes giving it a total of 14 torpedoes. The only exception was U-83, which lacked a stern tube and carried only 12 torpedoes.
Type VIIBs included many of the most famous U-boats of World War II, including U-48 (the most successful), Prien's U-47, Kretschmer's U-99, and Schepke's U-100.
On the surface the boat was powered by two supercharged MAN, 6 cylinder, 4-stroke M6V 40/46 diesels (except for U-45 to U-50, U-83, U-85, U-87, U-99, U-100, and U-102 which were powered by two supercharged Germaniawerft 6-cylinder, 4-stroke F46 diesels) giving a total of 2,800 to 3,200 bhp (2,400 kW) at 470 to 490 rpm. When submerged, the boat was powered by two AEG GU 460/8-276 (except in U-45, U-46, U-49, U-51, U-52, U-54, U-73 to U-76, U-99 and U-100 which retained the BBC motor of the VIIA) electric motors giving a total of 750 shp (560 kW) at 295 rpm.
The U-boat depict on the stamp is designed after a model seen on this URL, where it is given that she is U-99. http://models2u.co.uk/Shop/contents/en-uk/d68.html
15 December 1937 ordered, under yard No 593 by F.Krupp Germaniawerft AG.Kiel. Laid down on 31 March 1939 and launched on 12 March 1940 as the U-99. 18 April 1940 commissioned.
She made 8th war patrols in which she sunk 35 merchant ships, 3 auxiliary warships, 1 ship captured and 5 damaged.
17 March 1941 scuttled at 03.43 hours south-east of Iceland in approximate position 61N 12W after being depth charged by the British destroyer HMS WALKER. Of her crew 3 men were killed and 40 survived.
Guinea-Bissau 2005 550FCFA sg?, scott? 2012 600FCFA sg?, scott? (The stamps gives that the U-995 is showed but she is the U-99.)
Wikipedia and http://uboat.net/boats/u99.htm