Demo Lps with special labels started in the 1940's.. were very common in the 1950's and 1960's.. Used both as radio station fodder, and in-store demo disks. As time progressed the practice of printing special labels disappeared and by the 1980's no company made a special label for the DJ crowd. They mostly went to stamps on the jacket, or circles punched out in the barcode and cut-off corners on the cover.
A demo label LP is worth about 10% more than a normal LP. Though some with different color vinyl and special added features can be worth far more.
Stamped covers have no added value.