I think there is a slight problem with semantics here. When talking
about the headstock, spindle and the bearings, one refers to the
"front" or "rear" of the headstock, spindle and/or bearings. The
front is the end of the spindle where the tooling is mounted (like
chucks, face plate, etc.). The rear is the opposite end.
When the OP mentioned "left" and "right" shim packs, it sounded
to me that they had one of the older 9 inch lathes that used bearings
with two shim packs per bearing (one on each side of the bearing and
hence, a left and a right shim pack).