USP mags: steel or polymer?

[wild_wild_wes]

Do USP mags come in both steel and polymer for all models?

I just bought some HK factory mags for USPc 9mm, USPc .40, and USPf .40.
The Compact mags were steel, but the Fullsize was polymer. I also have a USPf .45, and it came with steel mags. Can someone fill me in on why only the .40 fullsize has polymer mags?

[Gendaito]

Full size 9mm mags are also plastic.

[HKP3000]

the first pistol in the USP line was the USPf in 9 and 40, they come with plastic mags.
All later engineered pistols come with steel mags (USPf45, USPc, P2000, HK45, P30)