Originally Posted by
esscee
They could be using the Boeing Line number as Reg, line number being the number given to each aircraft as it is manufactured on the Boeing production line and then is also specifically used for identification purposes in the manuals.
Boeing don't use Line Numbers as registrations (l/n 1786 was an NG built in 2005) and, as with all manufacturers, it's the serial number (MSN) that is used to identify individual aircraft in technical publications, ADs, etc.