[dswancutt]

Ken;

Here's how the part numbers break out:

3986872 - Volume Knob (Same as NOS knob on ebay)
3986829 - Tuning Knob
Here's where it starts to get hinky
3986873 - Tone Control
3980197 - Auxiliary speaker control (I think this is only on Blazers & Suburbans)

I check the sales brouchers and AM/FM stereo radios are not mentioned until 1980. I think this is why one on the "tone" knobs doesn't have an ear.

Hope this helps