J is "Special Order". Examples - 999 paint, 9999 interior, 300 Hurst & Superbirds too.
D is Driver Training car.
W is Dealership Rental car.
Q is as Culver's stated, you need to know the Y code from the broadcast sheet to know which one it is.
M is Sales Bank (Y16). Early cars with the M were Sales Bank / Show car. The early ones would have been used for some promotional purpose and then sold as a used car after its intended engaement was done.
Some of the letter meanings change from one year to the next also, the above examples are all from 1970 cars.
Edited to add the M.
