The "holy grail" of guitars would be the 1959 Les Paul Standard, though any from 1958-1960 with high grade of flames (that is, wood figuring on the maple top) is very collectible. And there are some others that will fetch lots of money (Flying V in Korina, from same period, for example).
In 1959, said guitar with case would retail $307.5 - that's equivalent to $3038.06 in current money / purchasing power.
These days, such a guitar will cost you minimum $200000. That's almost 11% annual ROI.
If it's a beautiful "case queen" that's been laying in its case for most of the time, you could easily get $500k. That's almost 12.5%.
Or if it's a celebrity owned, the sky is the limit - though your initial investment would have to be much higher, unless the guitar was acquired ages ago.
Same goes for Fender Strats and Telecasters from the same period (up to the early 60s) - though these are in much higher abundance. Nice ones can be had for $25k-$50k, which would give you a return right around, or above, the stock market.
But of course, few would have known this, back in the day. These guitars didn't become collectible until the 70s, and certain artist playing them helped the hype.
With that said, I don't think people that bought these as an investment, are too much into investing. It's a nice alternative/side investment that you can enjoy as you age - if you play guitar. They don't generate rent, they don't generate dividends, but that was never the point either.
So far, they've proved to be investments on par with the stock market - even beating the stock market if you have the right item. In fact, some of them have outperformed the stock market by magnitudes for the past 2-3 decades. Those $200k Les Pauls only sold for a fraction back in the 90s, so the majority of appreciation is a more recent thing.