Essentially, the Bombard was a Forgeworld artillery unit built on a Leman Russ hull from 3E/4E.
In the 5E
IG codex, they introduced the Colossus (without a kit) that was basically took the gun off a Bombard and stuck it to a Chimera hull.
When the next
IG codex came out and they became "Astra Militarum" and
GW instituted their "no model, no rules" policy at the end of 6E, both as a result of the Chapterhouse Studio's lawsuit, the Colossus got dropped from the Codex.
With 8E, both got merged as the "Colossus Bombard" in the
FW Index book