I've bought an S4 Showa rear shock on ebay, I have an M900, the million dollar question is, will it go on? It was very cheap so I'm not to worried about a no answer and it can always go back up for sale.
I await your answers and ridicule.
Get a ladder & straps and go try it out
It might 'fit', but the S4 used the ST style frame.
The linkage rates are different for the early 851 based, like your M900, and the later ST based frames so the later bikes used stiffer springs and different valving.
It might not be an 'upgrade'.
It's an inch shorter than the OEM shock on a 1st gen Monster.
And the piggyback reservoir will likely be an insoluble problem.
Thanks guy's
I had an idea it would be a problem, but at the time of buying I had no one to ask. Back on ebay it goes.