Bill:
Having unequal caster needs isn't a big deal - in fact it is also common. It's well neigh impossible to weld together a VW Pan with it's 150-200 separate parts and get it dead straight every time, and frames and suspension can take a beating over time on the road so VW designers took the logical way out - they allowed for adjustment post production.
Camber is easy - you just turn a big nut on the upper ball joints to adjust (although speedsters often need more adjustment, as Jack found out) and so Caster is easy, too, but you have to mechanically move the relationship of the top and bottom torsion tubes and, in turn, the angle of the vertical centerline of the trailing arms/spindle to get the correct geometry on the spindle/wheel to make it drive right.
It's been decades, but if I remember right you need to shim link-pin spindles to change camber, only the shims are, of course, smaller. It was enough of a PITA that VW went to ball joints in 1967 or so (and finally got rid of swing-arm rear ends in 1969, improving handling a LOT).
None of this is news - people have known about trailing arm steering geometry since the 1920's or so when it was first used (and then raced).
Still, it's not unusual to have different caster shims on each side. For instance, Pearl has an additional 3 degrees on the right, and nothing added to the left to get them equal, more than stock and feel right. Her difference is probably due to the doofus who built her (me). I've had her aligned three times - the first two guys told me that caster was not adjustable on VW front ends so I straightened them out (although they both had cool print-outs of the results). The last time I did it myself in Beaufort (a car club member runs a body shop with a laser alignment deck) and got it right on, except that I needed more camber adjustment than one side allowed so Jack got me a pair of "extra wide" adjusters that will go in when I get a chance.
On the frame horns, those are the two, big, tapered tubes at the rear of the pan which accept the transmission mounts amidship of the transmission. If the car has been hit on or near a rear wheel of if it's been jacked up repeatedly on the end of one horn, or the horn simply rusts or rots out, the horn can become bent which will throw off the suspension geometry and may require something different on the front end alignment to compensate.
BTW: Some VW drag cars that have been modified a lot in the suspension may require a LOT of added caster (and strange or "different" toe-in settings) to get them to stay on the track the way the driver wants it.
That's it.....I'm going back to working on the "new" old, rotted out pan, which is slowly becoming new again!
gn
Oh, and Jack - I learned this stuff the same way Larry did and my son, Chris, too; Get in there and do things, making mistakes and learning along the way. And also as a way to escape working 80 hour weeks in high tech. Nothing better after a day of high stress than to go home and beat on some metal, rather than beating on co-workers... ;>)
ciao