Update: So I changed my mind and bought a used '75 550F complete clutch unit off ebay. I pulled the clutch out of my spare '78 550 engine and dismantled & cleaned both clutches to check for differences and found that all the parts are identical except for the metal clutch plates, which are slightly thicker in the older clutch and thinner in the newer one with the dampener plate used instead of the third one. The older stack of plates when assembled is slightly thinner overall so the inner pressure plate sits a little further from the bottom of the basket than it does in the newer one.
I measured all the friction plates - with vernier calipres - and found one of the two disc B (outer plate) plates was thicker but the other 12 were all the same. I pulled the clutch out of the bike, dismantled it and measured the friction plates - they were all thinner than my spare 12- yay!. I measured the 12 clutch springs and picked out 4 that were slightly longer than the remaining 8 (turned out to be three from one clutch and one from my ebay clutch).
I pulled the shifter shaft out and easily replaced the seal. Put it all back together with the thickest of the 2 disc B's and 6 of the 12 spares. I didn't change the basket, but I did use the older metal plates (and no dampener plate!). I used two shimming washers under the circlip - I don't have a dial guage but it felt like a lot of in and out movement with only one, with two it still has a tiny bit of in out clearance- barely perceptable, but there still. I used the 4 longest springs and a new clutch cover gasket. To take up all the cable slack (aftermarket cable) I had to screw the adjuster screw in the clutch cover all the way anti-clockwise as I was running out of adjustment on the two cable adjusters. Is that a bad thing to do?
I took her for a 40 mile test ride and could not get the clutch to slip at all, so I'm gonna say mission accomplished! -and at a fraction of the cost of a new oem set of plates!
