Seal of 15 cents of the Olympic Games Amsterdam 1928

In 1928, the Portuguese Olympic Committee obtained the post office support to issue a seal to finance the participation of a national delegation in the Olympic Games of Amsterdam. In the so-called "Olympic days", from May 22 to 24, 1928, all correspondence was a franchisement with an additional postage stamp of 15 cents. Its failure implied the fine folded in the form of a seal of 30 centavos.
It was issued 1,800,000 seals, which rendered 90 contos of reis and allowed the sending of 29 men to Amsterdam. Portugal was present in 8 modalities: athletics, fencing, football, weighty, hopefulness, fighting, pentatlo and candle. Paulo D 'Eça Leal, Mário de Noronha, Jorge Paiva, Frederico Paredes, João Sassetti and Henrique da Silveira won a collective bronze medal in fencing (sword).
The drawing is from Roque Gameiro and is the first seal of Olympic theme by a non-organizing country of the games.
It was also the first time worldwide that the Olympic rings appeared in the philately. Since then, the rings are used in almost all Olympic seals, such that when they are missing, the term used to describe them is "no rings".