Okay, if you get this one, you're good.
From 1966, please enjoy the quite remarkable "You Used to Ride So High" by Abilene Motorcycle.
This is an unreleased demo, but as you can hear, it's a pretty amazing mashup of Mamas and Papas harmonies, a Buffalo Springfield-ish band track and the "dit-dit-dit-dits" from "Friday on My Mind."
The performers -- and there are two of them -- both went on to become quite famous a few years later, one as a producer and the other as a celebrated singer-songwriter.
I think it's great, and could have been a hit, although I will admit that the guitar solos -- which seem to have been played by Jerry Lewis --
--are kind of heinous.
In any case, if you already know who did it, please don't give it away in the comments.
Because I'm a nice guy, you can find a photographic clue to their identity here; if you think it makes it blindingly obvious (and I don't think it does) please don't give it away in comments either.
I should add that I had no idea myself of the existence of this until last week, when it rather blew my miniscule mind.
And of course, a coveted PowerPop No-Prize© will be awarded the first reader who correctly guesses the artistes.
[h/t Seth Gordon]