it was actually overweight and with a CoG aft of the allowable limit
There is a presumption that, when using standard weights, the loading folk/crew exercise some sensible oversight and abandon standard weights if such is patently appropriate.
Use of standard weights becomes more problematic as the number of occupants reduces (although not overly relevant in this case). This was shown quite clearly by John K's Australian study on the subject some years ago (which became the basis for current Australian standard weight rules).
Until that point, they were using the average weight of an American as defined in 1936
As I recall, the old 170lb figure dated back to an anthropometric study of US Army personnel in the 40s ?
Report
http://www.ntsb.gov/doclib/reports/2004/AAR0401.pdf is well-worth the read to emphasize just how fragile load and balance calculations are if housekeeping is not well-maintained during the process. One can only feel considerable empathy for the ill-fated crew which was clearly taken by surprise in a situation well outside the square ...