probably the first KM183 had the circuit board (always incl. connector) exchanged with a spare part. As the circuits are identical for all KM18x's, the circuits all have only one spare part ordering number, and all are fitted with a label from the KM184 ser.number pool (so also the spare parts have a serial number; and as KM184 are the vast majority of all KM18x's). To keep the original labeling and serial number, the original owner (or service-man) could have re-used the old label from the original pcb, but seemingly he didn't. Signalwise, the pcb's are all compatible / identical, from the beginning in 1997 to today. The mic with the #81489 should have a slightly lower noise floor, due to the newer circuit.
Best regards, Martin Schneider / Neumann Mic. Development