All the cucv's M1009 & all versions of M-1008, 1010, 1028 & 1031 vin #s start with a #1 as that means it's made in USA. They were made from 1983-86 or 87?. All of the vin numbers have 17 digits. Vehicles before 1981 had 15 digits or less. The first 11 digits of vin describe info. about the vehicle. As to the 4th digit being a "H" or "G" pertains to the weight factors of truck. If it is a "H" means that it will be either a M-1028, or M-1031 or a M-1010 Ambulance. The 1028 & 1031 were issue with the limited slip front Dana 60 called "Trac-Loc". It is not a pure "locker" like the rear end 14-bolt w/Detroit locker, instead it has the "clutch pak". However, the M=1010 Amb. has the "H" BUT it refers to the weight only and DOES NOT have the "trac-loc" but it is same as the M-1008 (open type diff)
To "CARNAC" The last 6 digits is what they call vehicle "serial #". It is where they assign a unique # to the actual frame of the truck. I have heard it is put on more than one place on the truck but I have only found one place and it only has the last 6 digits "stamped" on the top side of the frame without any indication that it is part of the vin # at all. On the truck & I assume on the blazer frame also (I just have not looked yet), it is located on as said on the rh side rail (pass). You will need a mirror to see it as it is on top of frame, where the frame starts angling down from the back of truck to the front. It will be right under where your butt sits. The area is only a couple of inches clearance so that's why you need mirror. You can feel it with your fingers, then use your mirror. You made also note some #'s stamped on side of frame close to where these are, but I think those are some GM part numbers.
Hope this helps.
Smokey