Frank - From what I can deduce reading through the
1966 Chevrolet Truck Data PDF, the 12 bolt cover rear axle was the regular C10 unit with 1 standard and 2 optional gear ratio sets. Only '66 K10 series trucks were equipped with leaf springs instead of the coil spring trailing arm rear suspension and used the DANA 44 rear axle with the 10-bolt cover.
Confirmation of the different axles and rear cover bolt counts was done by researching many resources plus Chevrolet Master Parts Books. I can provide more details if desired, but that would have to wait until another day as this one is closing out fast.
Below are just some of the resources I searched to help find this info.
