There was some other study or document from when the idea of USA Pkwy was first being touted by the developer, and plans to link it up from the then short stub off I-80 down to US 50 in Silver Springs. That document mentioned something about it being proposed as SR 805. This number was likely picked by the developer based on the linking of highways 80 & 50--805 would not fit in the numbering scheme for secondary (formerly "state aid") routes. This never got off the ground though, as I think NDOT was skeptical about taking control of the roadway when the developer wasn't planning to construct it to NDOT standards.
I also read the linked document which mentioned the possible SR 439 designation. With the USA Pkwy proposals having gained more traction, I think there is more thought on NDOT's part to take over the road, as a regionally significant highway. It's worth noting that this proposed number does fit in with the primary route numbering scheme--in this case, being a never-before-used number early in the Washoe County grouping.
The most recent NDOT highway log (July 2012 update) makes no mention of the route.