That is a very hard part to find... I know I just found a set. From what I could see in the bowtie bits catalogue on line, that is included in there refurbished heater kit. Put an add in the parts wanted,and talk to the usual venders.
Heaters were very rare accessories in the early 40s and 50s. A new truck was about $1500, and a heater was between $30 and $50 add on. People were frugal and wouldn't spend the extra dough, not just for chevy and gmc, but for all auto manufacturers.
