Yes,earliest prog is more difficult to pin down than psych.
I will now give you my submission for earliest lp to use the word “progressive” as genre… Please feel free to find one that beats it.
But, before I do, this is my logic:
1/ it would have to be UK since UK was where it flourished first (albiet not necessarily where it necessarily started - we will get to that.)
2/ it would have to be AFTER OCTOBER 1969 (Court of Crimson King)
3/ one would think that since we are talking genre, best bet would be THE EARLIEST COMPILATION to feature such bands.
4/ best bet would also be a largish label
This leads me to the various artists sampler lp put out by
Decca’s progressive and underground sub-label , ‘Deram’ :
WOWIE ZOWIE: WORLD OF PROGRESSIVE MUSIC
Okay, granted it says “music” not specific “rock” but half of this lp IS earliest prog bands: Moody Blues,Procol Harum, Genesis from the first lp, etc (look it up on Rate Your Music)
Harvest,Island and Charisma would have been other samplers to look at only that THESE ALL CAME AFTER THE DECCA ONE.
Another stronger contender - but one which I could not find a sampler lp for - was the other Decca offshoot, Nova . They were even more prog rarefied (Egg,Aardvark,etc)
Now I said it would have to be UK. There is one strange exception: although Court of Crimson King is BY FAR considered to be the Rossetta stone of prog, some do like to bring up the US band TOUCH and their lp which preceeded ITCOTCK by a short whiles.( I however consider it proto-prog)
The strange thing is that the Wowie Zowie sampler lp STARTS OFF with a Touch track!