I have just become aware of this difference because of recent eBay auctions. At first I thought I had a shot at RHS mirror(s), but they all appear to have a shorter arm than my LHS mirror. (Please note, I acquired this LHS mirror and cannot vouch for its correctness.)
First, there are at least 2 bolt patterns:
Staggered: one hole high on the flange, the other low,
Staggered/Centered: The forward hole is centered at about he vertex of the arm and flange itself.
unknown - there may be others...
I believe the S is 620 and the S/C is 510.
Second, there may be 2 arm lengths. The long arm is about 4.5" long the arm leading edge from the center of the ball joint to the longitudinal centerline of the flange mounting surface.
Third, there may be 2 arm angles: the short arm appears to be nearly perpendicular (90*) to the flange mounting surface plane, while the large arm is 30-40* from the mounting surface plane.
Regarding arm length and angle on the 620: It appears the factory mounting position is just in front of the leading point of the bullet "fin" which is not in the higher, top concave portion of the door panel - the mounting flange will not seat properly and will rock. It could be that the longer arm and tilt is used to position the mirror higher relative to its mounting point while also not letting the mirror stick out more like big truck mirrors.
Looking to purchase junkyard 5-spd for my 74 L18 620.
Looks like there is no inspection plate for visual inspection.
I think once, a long time ago, I once did a crude synchro test by spinning clutch on input shaft by hand w/trans in neutral and then jam into gear. If the synchro was broken, it would grind. Worn is another matter.