So, a bit more research on Sarna turns up TPTB never told us who started low-rate initial production of the Thumper Artillery Cannon in 3012. I'm going with Coventry Metal Works (leaving the Sniper, or more likely, Long Tom version for Hesperus II... now I feel compelled to come up with variants for those two too...), hence why a Phoenix Hawk chassis got the nod for this application:
3012 saw the culmination of over a decade of Lyran research and development into snub-nosed artillery cannons. Coventry Metal Works had drawn the short straw for production of the lightest version of the technology, and chose to apply it to a scout hunting variant of its popular Phoenix Hawk model, which they dubbed the PXH-1TAC. One executive unable to restrain their tongue at the unveiling of the design couldn't resist muttering "Chicken Hawk", and the name stuck. The weapon's weight resulted in the removal of all of the Phoenix Hawk's usual weapons, and a reduction of its armor protection. In the process, at least one engineer finally heard complaints from the field about the under-armored head. Even (or perhaps especially) in a scout-hunting role, the engineer reasoned the 'mech should be able to resist more than one lucky hit, and increased the head armor as much as they could. The argument of "resist a PPC hit anywhere" is what ultimately won the day.
