HR-101 - ADJUSTING THE HEATER CONTROL VALVE
HR-102 - HEATER DESCRIPTION - Pros and Cons
HR-103 - HEATER VALVE MOUNT - Bolts or Studs?
HR-104 - HEATER CONTROL - Attachment to Dash
  MG-203 - HEATER NOISE (csm)
HR-105 - HEATER TAG NUMBERS
HR-201 - os - HEATER VALVE IMPROVEMENTS - by Bob Muenchausen
HR-202 - os - MGB Heater Rebuild and Upgrade (easier on the MGA)
HR-203 - os - MGA Heater Rebuild - by Chad Pagel

Also see Electrical Function and Testing and in Electrical Restoration and Modifications.