From Warhammer 40k Wiki:
"The 122nd Cadian is an Imperial Guard unit from Cadia. They distinguished themselves in the Vogen Campaign against the Night Lords traitor legion, rebel cultists and the Planetary Defence Force. Among their officers were Veteran Segeant Hessel, and Captain Fane who was killed in action during the campaign against the Night Lords."
Dictator from Warseer states the following:
"The 122nd cadian was originally the 101st cadian. But after near destruction in the battle for Vogen it reformed its tattered remains into the 122nd. To represent this destruction of and reformation I will make the models seem very battle worn and almost gritty. And of course, the scheme chosen is urban looking because of the regiments past as urban combat specialists."
I don't know where he found this info but it sounds pretty good to me! Suits very well my decision to paint the vehicles with an urban camo and weather them very heavily.
- circa M40 - Winter Campaign on Vintock III
- circa M41 - the regiment participated in the Defence of the Imperial City of Vogen. Elements of the regiment under the command of Captain Fane faced renegade chaos space marines. At one point the Keystone of the position was mis-deployed when the Night Lord sorcerer, Asuramandos used warp majics to re-direct the defenders. The position was overrun in the ensuing battle.
- During the Defence of Vogen City at least one Leman Russ Executioner fought as the fourth tank of second squadron of the second company.
Overally there is not much info I could find but these little bits and pieces I did manage to discover were very useful.