68 barracuda 120 speedometer

There was a parts shortage due to a railroad strike. That's why the rally dash was placed in a few 68 Valiants. They used up whatever parts they had available. 68 model is supposed to have clip on speedometer cable, green dye circuit boards, dumb wheel radio, etc., etc...
So your speedometer wouldn't be correct for every 68. The one for sale here may not be either.
The clipped on cable is better engineering wise.