History of the oval sticker or euro sticker
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