63 trucks should have had self adjusters so yours has probably been changed. The drums may have been changed also if they don't have the knock outs. You can drill and file out a slot. I don't think that there would be a slot on the backing plates as the suspension components are in the way. You should be able to buy the springs and the self adjuster parts from a parts store.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine