I would imagine that the device plugged into the Xantrex inverter isn't happy with the modified sine wave output. I've used them in the past and 98% of everthing I ran with it ran fine, however, the other 2% wasn't happy. I would bet your 300 watt inverter that was working in this setup was a pure sine wave inverter.
It's a #6-32 x 5/8" stainless steel pan head phillips machine screw. The mating nut used is a #6-32 keepnut (integrated lockwasher). It can be fastened with a nut and separate lockwasher as well. Home Depot should have this as it is a fairly common size. Robert Graham Mechanical Engineer, Lectrosonics, Inc.