The two letters found on the case and dial of each Orient watch designate the movement, which is graded for accuracy. There seem to be only three major grades. This info is from a 2001 catalog. Thanks to Adriano from SCWF, who posted this info.
FC, EG : +10~-5 sec / day
FA, EX, FE, ER, EM : +25~-15 sec / day
PF, NR : +40~-30 sec / day