These service bulletins were sent out by Volkswagen as additions to the general heating service manual covering all models. They informed the shop about problems found with heating components or changes for new model years.
The scans on these pages were done by Russ Wolfe. You can find more bulletins for other heater models on his site at www.classicvw.org.
February 22, 1971: Warning light in temperature regulating switch
October 4, 1971: Ribbed heat exchanger and other changes (CA only)
September 4, 1972: Modifications for 1973 models
April 27, 1973: Fuel overflow pipe and possible ignition defect
August 19, 1974: Overheating switch for BA 4 heater
December 8, 1975: Temperature regulating switch