Raven says that Alexander's 'Mech has Hellstar PPCs in each arm and and a big Whirlwind cannon backing it up. The other main user of the Hellstar was the Striker, but that wasn't developed until 2571, long after the Davion Civil War.
That is more detail than given up thread.
Now that said... Hmm. I feel like you COULD get something like that via the Warhammer/Battleaxe chassis if you swap out the Missiles/Etc for an AC5.
PPC brand could be from a refit/repair change that resulted in the AC as well.
Possibly also paid for w/ a custom Endo chassis which was in use at that point.
Though really, I still think a customized Mackie is probably the easiest way to get that much firepower w/o huge changes.
The 7 Series used Standard tech v/s the Proto-Engine & was available for 50+ years at that point.
We have 2 canon examples but the fluff seems to imply there where many.
You could even call it the 7"R" to match the Marauder series.