RJ45 to RS232 With Power
This image from a well-respected purveyor of quality electronics (joke! joke!) may give you a start.



Source: Flexible scanner cable for Symbol LS2280.

If it is correct then the scanner can be used with RS232, OEM, USB and keyboard wedge cables.

External power supply negative goes to pin 4 which also connects to the PC port 9-pin ground, pin 5, and the power supply + goes to pin 7. All the others go to the respective pins on the RS232 connecter.

How did I find it? Google image search Symbol+Barcode+Scanner+LS1203+cable+pinout.