Cuenta:
Please consider that a clock that just shows time as from 01:00 -> 12:59 in AM and PM cycles does not need to display more than a "1" in the left most digit. What this means is that the total number of 'segments' that must be controllable is 2-for left digit + 7*3 for right most three digits. This means that it is possible that the 23 total 'segments' could be controlled from a total if 8 segment drive lines and three 'GND' lines as you have indicated.

It is possible that the Clock Chip used in that clock may have included one of the ALARM, COLON, and PM indicator LEDs into the mix with the "segments". It has been common for clock ICs to be designed to have the absolute minimal number of I/O pins but at the same time many had separate outputs to drive the AM/PM LED and an output for the colon LED so that it was easy to blink it at once per second.

It should be possible for you to easily make a lookup table to translate a desired digit sequence to the strangely mapped segment / gnd drive lines you found on the display.
Michael Karas