Here for recap
ref cn/fn
cn manufacturer construction number or serial number, ( at AIB e.g. MSN 033 )
fn manufacturer' fuselage linenumber,
Some Mfr use the cn in batches to classify production plant, acft type and such.
(Douglas had and Boeing uses numbers divided by a slash)
Their first number shows a totalling number of all orders,
the second part the fuselage-number of that aircraft type.
Klaus