Thanks for all the replies! I believe it was built in 76. It indeed currently has a Suntour 5 spd, but have an issue where it jumps from the 14t cog to the 22t cog and skips the 17t, forcing an upshift from 22t to 17t after the fact. I believe it to be from wear as I have tried all manner of derailleur adjustments. The current wheel I’m running is spaced at 126mm, so I believe the 7 speed should fit, might be a bit tight with the lowest tooth count cog.