my towbar has disappeared and is not replaceable on the equipment list. I think this may have happened when I installed rear triple lasers.
No such problem at me. I can not imagine any reason, these should work together.
Do you mean Towbar is not listed in F5 screen? Maybe you just got as much equipment that it is under "more"? In this case press the right arrow key.
In equipment shop (F3) eiher Towbar or Refund Towbar should be listed even in techlevel 1 systems, no exceptions coded. If none then probably you uninstalled Towbar oxp or broken it for example by editing a file within with notepad so try reinstall it in the expansion manager.