I found several dozen reels of 1/2 inch  black oxide 1.5 mil tape in my storage shed from at least 20/30 years ago. 
They are 10" pancakes with cardboard seperators with scotch printed on the master carton of 12 but no type number where the paste on sticker was is missing. 
On my Ampex 440 with "0" in i get  like -20/30 out. I tried re-biasing and altering the bias  has no effect on the output level....  
Any one have a clue why tape would act this way if stored for 20 years bulk ???
many thanks 
dnelson