NC 296: When the 50th anniversary of the interstate system rolled around, NCDOT put up a webpage describing North Carolina's history of their own interstates. Each interstate got its own pdf outlining facts about that interstate.
The I-26 entry (click on one of the highlighted dates) mentioned the corridor approved in Sept. 1946 as "Route 296"
It begs the question did all the interstates in North Carolina have route numbers in the pre-construction period in the late 40s-50s. There is precedent - Virginia assigned VA 4xx designations to various pieces of interstates as they came up for design and contruction.
The only other potential route number for North Carolina that I can speculate about is NC 293 for the northernmost part of I-85.
Obviously NC 296 was never posted on an actual road, nor does it appear on any maps.