The original cloth tape was dark on the exposed and adhesive sides. So far, all the gaffer's tape I've seen is white on the adhesive side. In that "little quirk", it may be considered not as original. A couple of years ago I contacted 3M about a dark adhesive side cloth tape but they didn't have that type. The rep was not sure if they ever sold a dark sided tape cloth tape. If you call a company's rep, mention you are working on a classic Mustang. That usually opens a lot of friendly doors.
Jim