Actually, one of the connectors on the rack into which the
transponder is installed has a set of contacts that are encoded
with the code for that aircraft. *Any* transponder plugged into
that position has the info for that specific airframe. Same kind
of technique is used for CVR, FDR and several other boxes.

I'm not talking about the physical location of the code.

I'm talking about the assignment. When your aircraft is registered
in the FAA database it gets a Mode S code (whether you have a Mode
S transponder or not). You can change the N numbers or the owners,
etc... but the Mode S assignment stays with that aircraft in the database.