It's not really a matter of a character representing a specific model, but the latter of the two threads will give you the more complete information. The VIN's all follow the rules of the 17-character VINs that were put into effect in 1981.
I don't think Mike ever followed up on the last thread to summarize, so I'll post my information:
For Chevrolet, the VIN positions are as follows:
1st = Country
2nd = Manufacturer
3rd = Type
4th = Gross Vehicle Weight
5th = Line & Chassis
6th = Series
7th = Body Style
8th = Engine
9th = Check Digit
10th = Model Year
11th = Assembly Plant
12th-17th = Sequential Number
Specifically for CUCV's, these positions can have the following values:
1st = 1 (United States)
2nd = G (General Motors)
3rd = 8 (Chevrolet Truck, Multipurpose Vehicle) <-- M1009
-OR-
3rd = C (Chevrolet Truck, Complete Vehicle) <-- M1008/M1008A1/M1010/M1028
-OR-
3rd = B (Chevrolet Truck, Incomplete Vehicle) <-- M1031 cab/chassis
4th = E (6001-7000 lbs GVW) <-- M1009
-OR-
4th = G (8001-9000 lbs GVW) <-- M1008/M1008A1
-OR-
4th = H (9001-10000 lbs GVW) <-- M1010/M1028/M1031
5th = D (Military Truck 4x4)
6th = 1 (10) <-- M1009
-OR-
6th = 3 (30) <-- M1008/M1008A1/M1010/M1028/M1031
7th = 4 (2 Door Cab) <-- M1008/M1008A1/M1010/M1028/M1031
-OR-
7th = 8 (Utility - Blazer) <-- M1009
8th = J (8-379 cid / 6.2L diesel)
9th = check digit
10th = D (1983) (Rare, if any except prototypes)
-OR-
10th = E (1984)
-OR-
10th = F (1985)
-OR-
10th = G (1986)
-OR-
10th = H (1987)
11th = F (Flint)
From my limited research (but fairly extensive experience and observation), the M1028A1 was only produced as an MWO that was applied to an M1031 to make a pickup from a cab/chassis. Therefore, it's VIN has a "B" in the 3rd position. The M1028A2 and M1028A3 are MWO applications to M1028A1 and M1028 trucks (respectively) to convert them to dual rear wheels. I have hear rumors of a few factory produced dually trucks, but have no way to confirm this. If there were factory produced dually trucks, I would believe they were probably M1028A3's.
Another note: M1010's are indistinguishable by VIN from M1028 trucks. Their GVW is 50 lbs greater and they have no Trac-Lok in the front axle, but you don't know it from the VIN.