Switzerland.
Dind ding ding! You get a cookie!
History of the oval stickers:
Well, back in 1969 the United Nations got together to create an easier way to identify the origins of different vehicles traveling in Europe and elsewhere, since so many license plates used overseas looked the same no matter what country you were from. With that in mind they came up with the idea of adding a sticker to your vehicle with a country code so officials could identify your point of origin. The original stickers were big...over 9 inches wide and nearly 6 inches tall! Almost the same size as a door on a Yugo! Here's the official text from the Conventions on Road Traffic from 1969 that describes the design of the stickers. Probably more than anyone 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
Grenada WG
St Lucia WL
St Vincent WV
Zanzibar EAZ
United States of America USA
Yugoslavia YU
Any State which has not already done so shall on signature or ratification of, or accession to, this Convention, notify the Secretary-General of the distinctive letters selected by that State.
4. When the distinguishing sign is carried on a special plate this plate shall be fixed in a vertical or nearly vertical position and in a plane perpendicular to the longitudinal axis of the vehicle itself. When the sign is fixed to or painted on the vehicle itself, this shall be done on a vertical or nearly vertical surface at the back of the vehicle.
I'm surprised Sierra Leone has one but Spain doesn't
