Maybe an I. tectorum cross?

Your iris looks like a sinosiberian (40 chromosome siberian type). The "name" I-31 suggests that this is a hybrid, or selected form rather than a chinese species iris. Chinese gardeners grow many iris that are not native to china. You would really have to discuss this with the person who provided you with the iris. There are many hybrids between the sinosiberians, and many crosses to the pacific coast group of irises, and other close species. If you take a look at the SIGNA photo database, you will see similar photos. I. delavayii is similar to yours, and comes in many forms because of it's ease of cross breeding withother types.

Here is a link that might be useful: SIGNA photo database