So from what I can tell it might be from an r6600 from 86-89 era. They were marked the ar15a2 sporter hbar. Chrome lined 1/7 threaded heavy barrels with a bayonet lug. The 6600 was from the era that colt did not block the sear location in their lower receivers.
I didn’t know where to start until you mentioned r6601. That helped me dig some more and other forums are all saying that the 6601 was ban era with no bayonet lug. The 6600 was pre ban with bayonet lug.
Looks like I now know what it came from. Like I said when I bought upper. It had an sp1 upper receiver. I assume with the sp1 upper and the shims behind the flash hider, someone slapped it together and made a bastard rifle.
Does anyone think this barrel has some value to anyone? If it’s not worth much, I’ll probably keep it and use it for my echo trigger. No use in me spending another $200 for a random off brand barrel to just beat the snot out of. Depending on value, I may entertain the idea of trading or selling it off.