While the exact contents of "v1.2.1" can vary depending on which entry in the series it refers to, it most likely represents:
: In some online communities, .zip archives with specific version numbers like this are shared as portable builds or modded versions of the game. About the Zombotron Series FiИ™ier: Zombotron.v1.2.1.zip ...
: An earlier entry in the Flash-based series. For reference, version 1.3 was a notable update in 2013 that added quality controls and fixed level-specific bugs. While the exact contents of "v1