What the heck did I buy?!

I acquired this 7” AR-9 barrel for my project to replace my short-horn 4”. The guy didn’t know the actual barrel threads and was listing them as 1/2x24 TPI. Very uncommon these days I told him so he sold it to me at a deal for me to investigate for him. It’s actually the common 1/2x28 TPI like a 9mm handgun or .22 cal. Just what I needed.

But what is the three prong lugs geared to? Roman thought it looked German to him. Anyone with a clue?