Trouble finding info on 2 specific codes in RPO breakdown

[87ChevyR10]

9MX0 - 4 spd auto trans ( i know its a 700r4, but there were different specific models and i cant find this code anywhere under any rpo code listing, closest resemblance is searching for "mx0" individually)
9J50 - this would solve my brake system dilemma once and for all. power brakes, light duty, JB3?
9GU2 - Ive figured out this one, so im in the process of planning out the rebuild/gears on the rear-end.

[87ChevyR10]

[dvdswan]

4th digit on the VIN should tell you what GVW and brake system you have. 

found these... from here.... http://www.crankshaftcoalition.com/wiki/GM_RPO_build_codes

MXO : MERCHANDISED, TRANS, AUTO PROVISIONS, O/D

J50 : BRAKE SYSTEM, POWER

JB3 : BRAKE, POWER, DISC/DRUM, 4600 LBS
JB5 : BRAKE, POWER, DISC/DRUM, 6400 LBS
JB6 : BRAKE, POWER, DISC, DRUM 7200 LBS
JB7 : BRAKE, POWER, DISC, DRUM 8400 L85
JB8 : 8RAKE, POWER, DISC, DRUM 10000 LBS
JC4 : BRAKE, POWER, DISC, DRUM 5600 LBS
JC7 : BRAKE, POWER, DISC/DRUM, 12000 LBS
JCB : BRAKE, POWER, DISC/DISC, 20000 LBS
JD5 : BRAKE, DUAL POWER, DISC/DRUM, 6400 LBS

[Captkaos]

Don't confuse the proceeding "9" as part of the RPO code, they are only 3 digits.
Most all of the 1987 RPO's are here:
http://www.73-87chevytrucks.com/techinfo/7387CKMans//Unit_Repair/ST_333_87_1987_Chevrolet_Light_Duty_Truck_Unit_Repair_Manual.pdf

MX0 4 Speed Auto. Trans, with OD. Requires MD8 (as it shows above)
MD8 Transmission - THM 700, Automatic 4-Speed
GU2 2.73 gears (as it shows above)
J50 is power brakes (as shown above)

As stated you need to look at your VIN to determine the brake system...