Stacking different ages of SW inverters for 120/240VAC output
An SW may be stacked with another SW of the same model number, regardless of age. All SW inverters have always been back-compatible in this regard. All you need is the SWI stacking cable to get the 120/240VAC output. http://www.xantrex.com/web/id/108/p/1/pt/5/product.asp
An SW PLUS is not recommended to be stacked with an SW, though.