[SS Tim]

Great Find!
Truely a "Hidden Highlander" that I have spent a lot of time researching and making people aware of.
The SPID codes and descriptions are correct for a 72 Baltimore truck. They didn't use the exception code suffixes as you noted, just three digit RPOs. Unlike most plants Baltimore didn't use a trim code either in most cases.
The Highlanders, being a special combination did!
In this case 618 codes the Orange Plaid Highlander trim combination upgrade of Z62.

YG6 codes the basic Highlander Promo Package "A" including BX2, B85, B93, DG4, PA1 & V37.

Of note is the E81 wood bed floor, a number of suspension options and even gauges.

Yet it has the standard 307 V8 and Column shift three speed (3OTT) trans!