Comment by sudahtigabulan

It has to do with the codes these keys (used to) emit in combination with Ctrl:

  Ctrl/H - Backspace
  Ctrl/J - Line Feed
  Ctrl/K - Upline
  Ctrl/L - Forward Space

Source - the ADM-3A operator's manual, page 3-5:

https://archive.org/download/lear-siegler-adm-3-a/Lear_Siegl...