The 2lb (?) steel rod inside the buttstock that reduces felt recoil.
The USH barrel was a spare for that receiver. According to him, he never even shot it. Lots of folks fit receivers, I'm just not one of them.
I wonder if mine has that. I might even have two 12 ga USH. Not sure. I have my main hunter for where shotguns are required, and I used to have a NIB spare, but I can't remember if I sold it. I've shot a couple deer with the USH, but I never really noticed the kick. It's a very heavy barrel, and I attributed all the weight to the barrel, but there may be some weight in the stock, too - I never thought to look, as it's a factory gun & I've never taken it apart. I'll have to check that out on mine.
I'm with you on the receivers - some do fitting work, but not me.
Best wishes selling it. They are a great deer gun for where shotguns are required, and even great in dense woods where shotguns not required, like NH, where you can't see more than 50 yards most of the time, anyway.