Bat Specifications
Harmon used primarily model S207s measuring 35" with weights varying from 31-34 oz. I do know that the also used D2, K55, and S100 with lengths as short as 34" on very rare occasions
Use Characteristics
I have owned more than a dozen Killebrew bats and viewed many more so I feel pretty confident sharing info on his bats. Furthermore, his use characteristics relating to pine tar useage have been well documented in the book BATS (Malta, et al., 1994).
Pine tar:
Pine tar on Killebrew bats is really a must. He was a notorious pine tar user. Like all things there are exceptions but I personally would not feel terribly comfortable with a Killebrew bat if did not have pine tar on the handle measuring 6-12" up the handle
Markings:
Harmon frequently wrote his uniform number "3" on the knob. The majority of really nice Killebrew bats that I have seen have had this characteristic. Again, like all things, there are exceptions. Furthermore, he would document the weight of the bat with a single number following his uniform number on the knob. For instance, if the bat weighed 31oz. you would likely find a tiny "1" following his uniform number "3" on the knob.