Can't tell by the pic, but if it's got a long tang that both pins would contact, it's from an old HK-91. If it has a short tang (tang meaning the part covered by the stock when assembled) the it might be from an old HK-93 or 94.
They were the one's designed without the 3rd full auto position for HK's semi guns.
I got the hk91 version of that lower on my ptr as well. Its a great option for those with bigger hands and like that look. Only real downside is it doesnt have those tabs up by the extractor so dirt can get in easier. But thats no issue if you're just range shooting, and if you ever go to a sand pit its just as easy to swap back to the polymer one for the added protection.
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