Tires needed for 1978 Schwinn Twinn

Hello Community,
I recently acquired a 1978 Schwinn DeLuxe Twinn Tandem 5-Speed. The bike is 100% complete and original. Every component looks like it will be usable after some cleaning and love. Except for the tires and tubes which had practically turned to dust. I am about to loose my mind trying to figure out which size tires I need. The 1978 Schwinn Catalogue stated that the 1 & 5 speed Twinn uses 26"×1-3/8" tires and the 10 speed Twinn used 27"×1-1/4" tires. The 26"×1-3/8" tires are too small for my rims. Here is all the info I can provide for my rims. The rims are stamped SCHWINN TUBULAR S-6, I think. The rear wheel is a 5 speed with expanding brake. The outer diameter of the rims are 610mm/24". Bead Seat Diameter is 598mm.