It's probably TO
That along with the casting date of D 7 7,(April 7th,1967),would mean it was assembled on April 21st,1967.
The VIN# of 149839 is applicable to a January 1968 assembled Chevelle-Elco.That means the engine sat assembled for 9 months before being issued into a vehicle.
That spells out to be a 1968 Chevelle or Elcamino built at the Kansas City Leeds plant,with the 396/375HP L-78 engine & Manual trans.
Kansas City Leeds plant only made Chevelles-Elcos-Station Wagons for Chevrolet.
The fact that it's on the later issued #854 block is a mystery,and I don't believe the Corvettes used the #854 blocks either and they started with #400001 in 1968.