There were a total of five versions of this game released. The original Japanese version, an American version, a European version, a version included in the Japanese release Konami GB Collection Vol. 1 (a compilation of older Game Boy games with full Super Game Boy support for each game) and another version for the European release of Konami GB Collection Vol.1.
The original Japanese version, titled Contra, was in black and white and has a stage select mode as a default feature in the game.
The US version is titled Operation C and has the stage select disabled from the game. Instead, the player has to input a code to unlock it.
The initial European version was titled Probotector, was based on the US version, but it had the main characters and some of the enemies replaced by robots, much like the previous Probotector games released in Europe.
The version included in the Japanese Konami GB Collection, is almost the same as the initial Japanese, only upgraded with Super Game Boy support such as a custom border and limited color.
The version in the European Konami GB Collection is called Probotector, but unlike the initial European version, none of the characters were replaced and the stage select is kept. The game is upgraded to full color with GBC-support.
In Germany, the player charakter is replaced by a cyborg/robot, like in every Contra-game in Germany. Also the name "Contra" is replaced by "Probotector"
Contra was voted #13 in the Top 100 Games of All Time poll published by Game Informer Magazine (Issue 100, August 2001).