We normally use C-CMD but someone have suggested that flying pilot should be using his side AP because of some wireing peculiarities:
Is XPD mode C always taken from same side ADC?
Does RH AP use RH ADC regardless of LH ADC being the active (normal)?
Does JAR require use of side matched ADC and APs?
( references, please )
Regards
[sp.]

Altitude Reporting (Mode C)
The “ATL RPT L/OFF/R” control is a three position toggle switch. It
selects the air data computer (ADC) source for the transponder to
reply with coded altitude information. In the L position the left ADC
is used, while in the R position the right ADC is used. In the OFF
position no altitude information is being used.



FCC/AIR DATA COMPUTER INTERFACE
Two air data computers provide air referenced data to the Autopilot
Flight Director System (AFDS). The left air data computer ( ADC)
supplies data to the left and center FCC and the right air data
computer supplies data to the right FCC. Each ADC transmits a source
destination identifier ( SDI) code to identify the source of the data.


Sorry, can't help you on that one! I'm just an engineer.

Boeing 767 engineers Training Notes.

Mode C: I can't remember having seen a mode C OFF posibility but I figure the text is valid for a L / R only system as well.

FCC/AIR DATA COMPUTER INTERFACE: Shame on me - if I had bothered I would have found that in my own books as well.

Now for the regulations:

JAR-OPS 1.872 Equipment for operation in defined airspace with Reduced Vertical Separation Minima (RVSM) (See JAA Administrative & Guidance Material, Section 1, Part 3, Leaflet 6)
(a) An operator shall ensure that aeroplanes operated in RVSM airspace are equipped with:
(1) Two independent altitude measurement systems;
(2) An altitude alerting system;
(3) An automatic altitude control system;
and
(4) A secondary surveillance radar (SSR) transponder with altitude reporting system that can be connected to the altitude measurement system in use for altitude keeping.
---
Re (4) if you use the RH transponder you must also use the R-CMD.

Thanks!

"FCC/AIR DATA COMPUTER INTERFACE
Two air data computers provide air referenced data to the Autopilot
Flight Director System (AFDS). The left air data computer ( ADC)
supplies data to the left and center FCC and the right air data
computer supplies data to the right FCC. Each ADC transmits a source
destination identifier ( SDI) code to identify the source of the data."

Note however, there is signal sharing between the FCC's. With dual sensor data (such as ADC's), in the air, the FCC's are forced to use a particular ADC depending on A/P or F/D selection. In a lot of cases, the Left ADC will be providing data. The Right ADC will be used if the Right A/P is selected or if the F/O's FD (only) is on and no A/P is engaged.

I seem to recall that the source select switches may also influence what feeds what.

Note that there are variations from aircraft to aircraft. It's best to ask the fleet specialists.

Regards.
NSEU