Most of the Howell calibers were made on 30-06 cases....not sure on this one.
Yes, the .450 Howell is based on a necked down and shortened .404 Jeffrey case, as were the original .375 Howell and the .416 Howell.
Ken also developed a .375 Number 2 Howell based on the 8x68mm S Magnum brass.
Ed