The rule of thumb for rubber/canvas power transmission belts was: Hp=WSP/2400Fwurth pointed out the same as I've found: the mechanical splices on an original belt just cannot stand up to drivebelt loads. Add the safety consideration of flying metal, and endless quickly becomes more attractive.
The Case Threshing manual shows 8" X 5 ply for 50HP. Of course, threshing is a fairly continuous load with potential for added abuse with a slugged cylinder.
I think I've seen more like 3HP per inch per ply, but may be my imagination.
The new materials, of course, more load per ply. This also equates to less forgiving on equipment from an overzealous throttle hand though.
Just from the small area shown, I'd think your belt deserves a proper repair, but probably semi-retired to some lighter loads.
Where W=width in inches, S=speed in feet per minute, P= number of plies. This was for new belting running on a 12” diameter or larger pulley. For pulley diameters less than 12” correction factors were applied, to reduce the horsepower rating. For a small pulley diameter of 10 “, the multiplying factor was 0.9; for 8”, 0.8; for 6”, 0.7; for 4”, 0.6; for 3”, 0.5. Other allowances were made for angle of wrap and nature of load.
So a 7” 4 ply belt running 2700 feet per minute on 36” pulleys would be good for 31.5 hp when new.