8 3/4 questions

Axles should be the same length. I don't know why someone put an adjuster on both sides but it doesn't need it. But as far as it hurting anything I don't think it could hurt anything having 2 as long as they are both properly adjusted to give approx. .010" end play. You know as much as there is to know about the 2 types of SG units. 68 was the last year for the clutch type SG so it's a 68 or older unit (although it could have came out of a real early 69 as Ma used up left over parts that way). I believe Moser sells plates. Dr. Diff might also sell them to and he'd probably be cheaper. Heck he might even have a used one he'd sell you cheap. If you don't find any at either of those places let me know as I have some parts out in the shed and might have a used one, will have to look. Only problem about changing that plate is you will have to cut the bearing retainer off to change it and they don't sell retainers separately so you will have to buy a new axle bearing. If your axle has a good Timken bearing on it you can buy an el-cheepo Master Pro bearing from O'Reilley's or Autozone for about $12 and just use the retainer with the good Timken bearing. Then the retainer has to be pressed on. Since it backed off I'd suggest checking the seals real good cause a lot of the time when they back off it damages the axle seals.