These cards were designed by the Thorburn group here in Minneapolis. They are letterpress printed with a silver ink on both sides of a custom duplex black and white stock. Custom pasting of a duplex of paper stock is a good way to make a project with look and feel unique. It's very often the best way to get both the colors and stock thickness desired for a project. And lets face it - most of the stocks available from paper companies as pre-duplexed options are pretty fugly colors and/or texture combinations. This stock is Wausau Royal Complements Eclipse Black 100lb cover pasted to 100% cotton Neenah's Crane Lettra Flo White 110lbC. For production, these two papers were pasted together, then letterpress printed.
As an end note, I worked at Thorburn several years before the jump to full time operations of Studio On Fire. Check out the Thorburn site here for some solid design work.