In the OP's defense (not that he needs any) ...
Those knobs are not labeled; their function is not necessarily immediately apparent. The manual is no great help, either. In the manual for my old 1977 Cherokee 140, the reference was buried at the end of a longer paragraph, even less apparent than in the example posted above.
This is as close to an illustration as that manual gave you:
View attachment 63433
I'm not sure when those knobs were added to the heater ducts. The 1969 manuals, and Piper's 1971 guidebook for us sales demo pilots, don't mention them at all.
It was not an unreasonable question. I'm sure I probably asked the same question of somebody way back when.