Canon EOS-1D X Mark III Advanced User's Manual page 397

3.
Check and change the lens information.
(1) Registered number
Displaying lens information
Press the <
The lens name is displayed with a 10-digit serial number. When the
serial number is displayed, select [OK] and go to step 4.
"0000000000" is displayed if the lens serial number is not recognized,
and in this case, enter the number manually.
For details on the asterisk "*" preceding some lens numbers, see
serial numbers.
Entering serial numbers
Select the digit to be entered, then press <
Enter the number, then press <
After entering all the digits, select [OK].
> button.
397
> so <
>.
Lens
> is displayed.