The socket in your picture is a '25 pin D socket' it is not a standard socket for model railway use. It originates from the 1980's [and before] and was a common component on IT (Information Technology) hardware for Data Communications. There are also 9 Pin and 15 Pin variants of this socket too. The 15 Pin version can usually be seen on old computer graphic cards and monitors for VGA graphics [now pretty much obsolete these days]. The 9 Pin varient was usually seen being used as Computer RS232 COM ports [pre dates USB].
The wiring of your 25 Pin socket will be completely custom and designed by the person who originally built your layout board. Only the original builder will be able to tell you its function and how it is wired. It will not be compatible with any similar socket as found on commercial model rail hardware such as the HM2000.
Model railway builders have used the D type sockets to allow them to build a layout that has an element of modularity in it i.e the ability to join boards together or join / disconnect a control panel or other control elements from / to the layout. Control elements such as point operation switches and wiring. Exhibition layouts for example might use these connectors. The Pins on the connector are quite thick and robust so are convenient for constant plugging and unplugging, they also include locking screws on the plug that once done up, prevent the connector from being accidently pulled apart [note the fixed threaded nuts at each end of the connector in your image ... these mate with the threaded screws on the plug].
The bottom line, is that in the absence of the original builder, only YOU can determine the function of the socket by tracing out all the attached wires to it and see what they connect to. Then draw out the circuits on paper to make sense of it.
The standard numbering convention for the D25 Male Plug is thus:
Note that on the socket, the female socket will start numbering from Pin 1 on the left [the image is for the male plug]. Or in your case, the bottom right of the socket as it is upside down in your photo.