Bock's standard size 6 nib is the type 250.
The type 220 you mention is only available in solid gold, and getting hold of them is like trying to find hens teeth. We did manage to get one as a special order for a customer recently, but it took Bock almost a year to supply it to us. It uses the same housing as the type 250, but there is no real advantage in having one unless you want a size 6 nib that is a tiny bit narrower across the shoulder than a type 250.
Type 250 is available in polished steel, as well as gold plated and two-tone plated versions, along with various precious metal versions, principally solid gold, solid palladium and solid titanium. As I say, it's Bock's standard size 6 model, and we carry all versions of them in stock, in all point widths, including the precious metal ones.
I hope that helps