The Modification Numbers are sequential from the first aircraft modified, # 1 through the last, # 43.  The serial numbers are the production numbers of the G II to which the G III wings were fitted.  The result was called the G II-B.  No G II-B's were built as new aircraft.  All of the new aircraft built with this wing were G III's, which are about 2.5 feet longer.  The DC G III's were basically the same as the G II-B's systems wise.  The windshields were different, and the cockpit gauges and displays were modernized, but the systems did not change much until the AC G III came along late in the G II / G III program.