Graeme -
My suspicion is that the reference in your first scan is a typo. The XF-92A apparently originally carried the XP designation and was changed during the redesignation process in the late '40s early '50s. The F-82 was the Twin Mustang and it also originally carried the P designation.
These two scans are from a little picture book titled X-Planes and Prototypes by Jim Winchester.
Hope this helps some.
Gary