Cogwheels

On August 9, 1969 Singapore issued a set of six stamps commemorating the 150th anniversary of its founding. The 15c value (SG #121) of the set shows a gear outline below a view of an aircraft over the city's silhouetted skyline.

Here is Republic of Korea's July 4, 1983 70 won stamp (Sc #1346) commemorating the Science and Technology Symposium hosted that year by Seoul.

This .45 bolivars value (Sc #1059) of Venezuela is from the December 18, 1973 "Jobs for the Handicapped" three-stamp set.

On March 10, 1973 Republic of Korea issued this 10 won stamp (Sc #857) commemorating the 10th anniversary of Labor Day.

This February 10, 1976 20 won stamp (Sc #1014) of Republic of Korea commemorates the 10th anniversary of Korea Institute of Science and Technology (KIST), the country's first multi-disciplinary scientific research institute.

This June 1986 pair of 100 franc airmail stamp (Sc #C356) of Benin is an overprinted, unfinished Dahomey stamp of 1970..

Cancellation reads Cotonou Aeroport 9.8.86.

Here is Egypt's October 14, 1970, twenty milliemes stamp (Sc#840) commemorating the International Organization for Standards's (ISO) 1st World Standards Day.

On March 17, 2000 Canada issued the 4th series of its Millennium Collection stamps as a minisheet of four 46c "Canadian Inventors" arranged as a se-tenant block. One of these stamps (Sc #1832a) depicts George Johann Klein (1904-1992) who created a wide range of devices reflecting his mechanical engineering expertise.

From wikipedia, "(He) is often called the most productive inventor in Canada in the 20th century."

This June 20, 1973 50 lira stamp (Sc #1107) commemorates the 50th anniversary of the State Supply Office.

Here is Ecuador's April 10, 2008 three U.S. dollar value stamp (Sc #1927) commemorating the 80th anniversary of the Chamber of Industry of Tungurahua.

Mexico
Century XXI Exhibition
July 6, 1962
The design shows Space Needle (Seattle, USA) and cogwheels.

On June 10, 1956 the DDR issued this 20 (pf) stamp (Mi #527) commemorating the 10th anniversary of 'Volkseige Betriebe" / state-owned companies. The "VEB" emblem is shown.

This 75 peso value (Sc #1433) of Mexico is from the February 7, 1986 three-stamp set commemorating the 50th anniversary of National Polytechnic Institute. The three different values were printed in a traditional sheet in a horizontally se-tenant fashion. Stamp depicts the IPN seal, which includes a gear.

Here is Chile's November 4, 1968 two-stamp set (Sc #s 372-3) commemorating the 40th anniversary of the Chilean Automobile Club. The Club's emblem is shown on the stamps.

On February 23, 2005 Romania issued this 21,000 leu stamp (Sc #4699) commemorating the centenary of Rotary International in a mini-sheet of four (2 x 2) format.