When you have to replace or install an APU generator on the -280 APU, there may be 2 different types of generator attachment studs mounted on the APU gearbox, a short (22 mm) and a long (32 mm) type. This may require also different nuts to attach the generator on the studs, due to the fact that the high nuts are too high for the short studs. (not enough threads coming through the nuts).
For short studs (22 mm), you can use LOW height nuts p/n NAS1805-6 or BACN11Z6CK or 624AG6HD
For long studs (32 mm), you can use the low nuts but also the HIGH height nuts p/n 624AG5HD or BACN10GW6A
See IPC 24-21-21-02 or CMM 49-10-21
See pictures below of short and long studs and nuts.