What a sweet find!

If it had been a 5 speed freewheel, wouldn't it have had a cable stop for the RD on the chain stay? But with that said, if it had been a 3 speed SA IGH, I would have expected rear facing "track" drop outs. Looking at your dropouts, are they Campagnolo? If so, might they have been replaced when the re-paint and pinstripes were done in the early-mid 1960s? So many mysteries!

I believe I still have a set of Superior 3 speed SA IGH wheels from the early 1950s that IIRC are 26" S7s (I'd need to dig around in my basement). My guess is the front would match the fork spacing. The hub is low flange. I'm certain we could work out a trade, etc., if you are interested.