I tend to disagree with the bullet puller theories because the guy that gave me the old ammo that contained these strange Winchester cartridges/bullets was not a reloader. He didn’t even have a loading press let alone a collet puller. He was a state wildlife officer and was issued ammunition by his agency; he didn’t have a real need to handload his own ammo.

I assumed that the linear striations are from the six stab crimps on the mouth of the bullet jacket and that the crimps were intended to mechanically bond the soft lead core to the gilding metal jacket.

Hopefully Winchester can explain why the lead slug is trying to extricate itself from the jacket.