Originally Posted by
icemilkcoffee
120mm was for 5 speed freewheels and occasionally 6 speed 'Ultra' freewheels (it's a 6sp freewheel designed to fit in the same space as a 5sp freewheel). Regular 6 speed freewheels typically went with 126mm drop outs. If the freewheel is a british threaded spin-on, you can replace it with a modern 7 speed freewheel with HG style cogs. If it's a Maillard type then you are out of luck.
But to the OP, the best way to learn about older bikes, is to read Sheldon Brown's website end to end
Awesome! The external resource is much appreciated. I hadn’t realized all the nuances of different part sizes between makes/countries. It’s amazing how much I’ve already learned without yet getting my hands too dirty!
What a wonderful community this is, all the help and suggestions are much appreciated!