The TECHNICAL SPECIFICATION FOR GNSS FLIGHT RECORDERS includes these
definitions and requirements relating to the serial number:

"FR Serial Number (S/N) - For an IGC FR, a set of three alphanumeric
characters allocated by the manufacturer (unique tothat manufacturer),
identifying an individual FR. The S/N appears in the first line of the
IGC file. If identification of an individual FR is required, the S/N
should be prefixed by the manufacturer's name and followed by the FR
model and type."

"Serial Number (S/N) – The three-character Alphanumeric identifier for
an individual IGC Flight Recorder. This appears as a three-character
group in the name of IGC files and also in the first line of the IGC
file. Although the word "number" is used, a more accurate term would
be "numbers and letters". Non-IGC recorders may have different S/N
systems."

"2.2.6.1 Identification Markings. The case of an IGC recorder must be
permanently marked with the name of the
recorder manufacturer, the type and version and the three-character
alphanumeric serial number (S/N) of that recorder. Where a display is
available, the recorder make, type, S/N and version must be shown on
that display for at least 10 seconds before it changes to another
presentation, so that it can easily be written down by an OO before
flight. Other details on the display are optional because many are
included in the IGC file header record. Where a recorder is designed
to be mounted in an instrument panel, the above data must be easy to
see and record from the cockpit without having to access the back of
the panel."

"3.10 First lines of IGC files - Serial and Version Numbers. The first
line of the IGC file contains a manufacturer
identification and a three-character alphanumeric Serial Number (S/N)
unique to that Recorder. The H- (header) record of the IGC file must
also include the version number of hardware, firmware, and details of
the GPS receiver and Pressure Altitude sensor (see Appendix A paras
3.1 and 3.2)."

"A2.5.3 FR Serial Number (S/N). For IGC FRs, the three-character S/N
must be used in the A-record and be imprinted on the case of the
recorder unless the case includes a display which includes the S/N.
See the definition of Serial Number in the Glossary."

For the two Cambridge flight recorders I own (302 and 25) the FR S/N
is visible externally on the unit and is not changed when the unit is
serviced or repaired.

Is there any legitimate circumstance in which the FR S/N assigned
during manufacture would be changed when the unit is serviced or
repaired? Such a change would appear to conflict with - "The case
of an IGC recorder must be permanently marked with the name of the
recorder manufacturer, the type and version and the three-character
alphanumeric serial number (S/N) of that recorder. "

Reason for the question is as follows:

Power FLARM brick FR S/N is identitifed by a marking on the unit
seal. My unit was defective on receipt and was repaired by the US
repair center. It was returned with a new seal covering the cut
original seal, and that new seal bears a different FR S/N.

I have no way of knowing how the unit is identified in the log file as
the flight recorder function is not yet enabled.

thanks

Andy (GY)