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This time I cannot find the rules for that heirloom battle cannon. I think it was in one of the campaign books? If so, which? And is it even still a valid choice in 9th? I suspect I am soooo late on this it's gone.

You might be thinking of the Hammer of Sunderance relic, from the Emperor's Fist Tank Company specialist detachment (published in Vigilus Defiant).

Specialist detachments technically still exist, but they aren't legal to use in GT2021 missions, so aren't really used anymore.

Specialist detachments are still allowed in Eternal War missions, but those are generally seen as just inferior version of the GT missions, and so aren't commonly played.
They 'might' still work in crusade games, although I'm a little fuzzy on how crusade interacts with specialist detachment relics.

There is a new version of it in the Cadia supplement (actually slightly improved) from War Zone Octarius 1.

It's Cadian only, and a relic for a TC with Battlecannon.

Thanks. That is exactly what I was remembering. Again, I appreciate the time.