another engine ID question
 

Sorry for probing a topic that has been covered a million times, but, I can't find an answer anywhere. I have a 250 chevy engine in my '66 gmc truck. That is what gmc offered; along with the v-6's. My casting number is 3877178 which is correct (either 230 or 250) but since mine was built in Sept of '65, its a 250.
Although I didn't pull the valve cover to look at the casting #'s on the heads, I have read up on the 250 for 1966 multiple times.
On the block pad behind distributor I see F0927 (Flint, Sept. 27). My casting is I 24 5 (Sept 24, 1965). The sequence # on my vin is 3384 (early build/Sept). This may not be the original hammer though all these numbers work based on an early build.
MY question: Stamped under the F0927 I see TAT 02877. What is TAT? I have found that TA is inline 6 with manual transmission. But, can't figure out the second T. In 1979 GM used T to designate truck. But I can't find that info for 1966.
This search has been off and on for a few months. I would greatly appreciate someone's help. THANKS