Where did the "euro" sticker originate? In 1969, the United Nations agreed to create an easier way to identify the origin of different vehicles traveling through Europe and elsewhere, since so many license plates used overseas looked the same no matter the country of origin.  With that in mind they came up with the idea of adding a sticker to all vehicles with a country code so officials could identify the point of origin.  The original stickers were over 9 inches wide and nearly 6 inches tall.  This is the official text from the Conventions on Road Traffic from 1969 that describes the design of the stickers.  Probably more than you ever wanted to know about oval, euro stickers!

 ANNEX 4. DISTINGUISHING SIGN OF VEHICLES IN INTERNATIONAL TRAFFIC

1. The distinguishing sign shall be composed of one to three letters in capital Latin characters. The letters shall have a minimum height of 80 mm. (3.1 in.) and their strokes a width of 10 mm. (0.4 in.). The letters shall be painted in black on a white ground of elliptical form with the major axis horizontal.
2. If the distinguishing sign is composed of three letters, the dimensions of the ellipse shall be at least 240 mm. (9.4 in.) in width and 145 mm. (5.7 in.) in height. The dimensions may be reduced to 175 mm. (6.9 in.) in width and 115 mm. (4.5 in.) in height if the sign carries less than three letters.
As regards the distinguishing signs for motorcycles, the dimensions of the ellipse, whether the sign is composed of one, two or three letters, may be reduced to 175 mm. (6.9 in.) in width and 115 mm. (4.5 in.) in height.

3. The distinctive letters for the different States and territories are as follows:

Australia   AUS   Austria   A   Belgium   B   Belgian Congo   CB   Bulgaria   BG   Chile   RCH   Czechoslovakia   CS   Denmark   DK   France   F   Algeria, Tunis, Morocco, French India   F   Saar   SA   India   IND   Iran   IR   Israel   IL   Italy   I

Lebanon RL Luxembourg   L   Netherlands   NL   Norway   N   Philippines   PI   Poland   PL   Sweden   S   Switzerland   CH   Turkey   TR   Union of South Africa   ZA   United Kingdom   GB   Alderney   GBA   Guernsey   GBG   Jersey   GBJ   Aden   ADN   Bahamas   BS

Basutoland   BL   Bechuanaland   BP   British Honduras   BH   Cyprus   CY   Gambia   WAG   Gibraltar   GBZ   Gold Coast   WAC   Hong Kong   HK   Jamaica   JA   Johore   JO   Kedah   KD   Kelantan   KL   Kenya   EAK   Labuan   SS   Malacca   SS

Malaya (Negri Sembilan, Pahang, Perak, Selangor)   FM   Malta   GBY   Mauritius   MS   Nigeria   WAN   Northern Rhodesia   NR   Nyasaland   NP   Penang   SS   Perlis   PS   Province Wellesley   SS   Seychelles   SY   Sierra Leone   WAL   Somaliland   SP   Southern Rhodesia   SR   Swaziland   SD   Tanganyika   EAT

Trengganu   TU   Trinidad   TD   Uganda   EAU   Windward Islands     Grenada   WG   St Lucia   WL   St Vincent   WV   Zanzibar   EAZ   United States of America   USA   Yugoslavia   YU