This is a single 4.25" x 5.5" notecard with an envelope. It is a folded card with a design of a mule on the front and a blank inside for a note.

These cards are based on an original linocut print featuring a a group of wild equines spotted in the Nevada desert. You often see both wild mustangs and burros but these looked like mules to me. It certainly happens in the wild but I believe rarely.