These weapons are pretty similar, right?
What Victis ends up building is pretty crude, but the materials they used weren't all junk. The handbrake (grip), mannequin foot (stock), and gas canister (ammunition) are all clearly scavenged trash, but the rest of the weapon is all its own part and it's a curious one. I believe this fourth part is a destroyed wonder weapon developed by Broken Arrow.
We know from a map found in Alpha Omega that Die Rise was a weapons testing site used by Broken Arrow, and Alpha Omega also confirmed that the U.S. siezed all the research conducted by Division 9 after the war. This would likely include, among other things, the blueprints for the KT-4.
So with Die Rise being a weapons testing site for Broken Arrow, who likely inherited the blueprints for the KT-4 from Division 9, it'd make sense that they were constructing their own version of the weapon there. The 4th mystery part of the Sliquifier would then be the broken remnants of this weapon, found by Victis and then reassembled into the Sliquifier.