[Rod]

The dash space is a state identifier if needed in certain styles of GMC vins for trucks that we destined to be sold in Pennsylvania. Hence if the truck was destined to be sold in the state of Pennsylvania there would be a T or K in the vin instead of the dash. You would also have a SPID RPO option number of Z55 for the T or Z74 for the Z to reflect the truck was made to the states requirements at that the time of manufacture.
If it is just a - (dash) in the vin at that location then it wasn't made to be initially sold in Pennsylvania. These typical dash style vins only appear in vins for GMC that use the 13 character long vin typically and in the years of 1970 and 1971 typically.

GMC also used the 34 instead of 4 to identify a pickup box body style in there vins in some instances which made the vin longer, which messes with the online chev decoders answer.

There are odd vins in a number of years in the GMC line. An example would a be a 67 GMC which could start with C9S basically meaning a conventional truck made in Canada with a 6 cylinder. The 9 identifed Canadian built in the Oshawa truck plant in Canada. GMC's were also built in some USA assembly plants also and most used the typical easily decodeable vin.
On some GMC's you have a letter at the end of the vin A, B or C, which reflect GVW ratings. Hard to make a GMC decoder like a Chev one as GMC styles of vins vary in different years.
I got them figured out though, this should help others wondering what's with the letters and dashes and numbers which seem extra in GMC vins. Rod