engine serial number?

looking at the numbers on the motor that I got out of a 81 D150

head part number 4104362, so that is a 79-80 mechanical lifter head
block casting number is 2806830-7, that makes it a 68-75 RG block

the serial number stamped on the pad behind the alternator has

4225 T 312

I'm confident the 225 means it is a 255
what about the 4,, would that mean 1974?
any idea on the T
is the 312,,, March 12th?
or perhaps a julian date, 312 day of the year?

thanks