Wheel Vintiques Magnums bigger than 15" are billet as far as I know. Specialty Wheels, Portland OR makes steel Magnums in 17". They are a 15" Magnum center with a hoop that flanges out to make the extra outside diameter. Some folks really like them, others find them proportionately "off". They are high quality, nice detail painting, and flawless chrome.
I grew to really like their look, but went to the Vintiques billet Magnums in 17" because the steel ones weigh a ton. I still have them, but they're probably going to the swap meet at our local Mopar show this weekend. Heres a pic of the Specialty 17s.