I've saved you some leg work!
My inquisitive mind got the better of me and I couldn't resist finding the answer!
Here you go:
Ohlins XTR fork springs are indeed different and longer. Only available in 5.0N - Part no. 8709-50
Ohlins XTX and XTZ are the same, and interchangeable, but with spacers used on the Z. Available only in 6.0N (there used to be a 5.0N spring, but it was/is discontinued) - Part no. 8716-60