For reference>>
Not sure how many bells are OEM from build date but 3" diameter black finish bell assemblies for Gottlieb Used for 1 point score sounds, 3" diameter bright plated bell assemblies for Gottlieb are for usually 10 or 100 point sounds,5" diameter bright plated bell shell for Gottlieb usually 100 point sounds.
The 1959 Sweet Sioux by Gottlieb i have has the three bells i just described.
It also depends on if you want to have all three seperate tones?
You can also wire it for both 10/100 points using the same bell too. Just depends on the tone you want. Steve Young(PBR) may not have them in stock like you said but he should know what bells were in from the factory(maybe send an e-mail inquiry)
If you have a manual or schematic it says how many and which size also.
The 5" bell was in the cabinet the others in the backbox.