I believe the D means it's gear drive and the T means 4x4.
So 2650D = Gear Drive, 2wd, 2650DT = gear drive, 4x4
However, if your 2650D is a 4x4, then the D & DT are synonymous.
No that's not really true.
D and DT mean the same thing, means 4wd
F means 2wd
There is no letter designation for gear drive
There is a designation for all other drive types, like HST, GST, WET
A L2650 gear drive 2wd would be simply L2650F
A L2650 gear drive 4wd would be either L2650D or L2650DT
A L2650 gear drive 4wd with a wet clutch would be L2650D-WET or L2650DT-WET.
A L2650 Glide shift 4wd would be L2650D-GST or L2650DT-GST
They never made a GST or WET in 2wd
And they never made any with a HST.