If I'm not mistaken, I believe the '73-75 Blazer/Jimmy rear window channels are unqiue because of the framless doors and no one makes a a replacement for them. I think the bolt hole is higher on the pickup/'76-up blazers, or something like that. If I rememeber LMC has a note, I could be wrong, but I thought I saw it, a note saying something about "modification needed" or something like that for the '73-'75's. LMC loves to do that. They use tailgate pickup latches for thier blazer tailgate latches, put a different part number on 'em and a note saying "May need modification" so you think your getting something specific for your truck, and low and behold I'm grinding off these little tabs with no way to prevent the metal shavings from penetrating the nice new nylon bushing that makes the latch work so nice, and the tabs the prevent installation into a Blazer tailgate...anyhow..I digress...that's another story...
I have a '73 I'm in the middle of restoring.
I just bought the rubber for the channels from GM for a '91, chiseled it out of my old '73 channels and glued in the new '91 rubber to the '73 channels. So since you already have new channels, just swap the rubber from the new ones to the old channels that have the right mounting locations.
BlazerMan