I have a 396 with the following Code: TO321EKH
I think the Code is:

T is for Tonawanda
03 is March
21 is the Day
EK is 396 / 325 Horsepower with Powerglide
H ????

What does the H stand for? Could it mean it was equipped with a Factory Holley Intake and Carburetor? Holley is stamped on the intake.

Any help would be greatly appreciated.

EK suffex was used in 66, 67 and 68.
R= Rochester carb
H= Holley
Not all got the extra suffex letter.
Is your block dated for 66 by chance? What is the casting #?

The 396 is in my 66 Chevelle SS and is original to the car. The Chevelle is away for the winter, so I will check the casting number in the Spring. The H is what always confused me. The build date on the cowl tag is 03E.
Thank You for the help.

Note

Just because it came with a Holly carb does not mean its anything special. Holley, Carter and Rochester were normal carb suppliers at the time and they coded the engine with R or H because the manifold differed between Holley and Rochester bolt patterns.

Some small block 327's were the same.

Its is believed they used multiple carb makers because Rochester could not supply all the plants as they did not have the production capabilities, so Chevy had to outsource.