the book your talking about list all the models and specs ? part number on the bottom 363 ? that book was pre mxA and covered all the 73 model including the At1 etc, anyhow it cold be they found the originals springs were too soft as folks started to ride em more and they did an upgrade on the specs or did an alternate part to pick up the problem infield.
so the number in the book may be the first part number then it subbed out into other over the next year or so, may have even picked up the b series i dont know, try them all
i would have though the longer or stiffer springs would have increased pressure on the plate thus giving more force on the disks. The old springs may be a differant tension or just lost there tension and not holding force like they used to. put in the new springs and report, it has to be somthing to do with the pressure on the plates or the way the plates lock into each other ( such as old contaminate in the oil making a latent slip.)
if the full pressure is on the corks and it still is soft to engage it can be worth a look at how the arm is actuating as in back the daylights out of it, so there is no clutch at all, then see if it still moves under power.
From my understanding the rest of the bits past the clutch are all gear driven with no movement, therefore the only place stuff can pick up movment is in that clutch boss area, so we have to guess its an adjustment, a plate or lack of pressure thats not quite right.
