Sorry this reply is so long in coming but I've been posting over at the other RTV900 site at tractorbynet for the last year and didn't realize there was another site I could be hanging out as well.
Anyway - you can tell which springs you have by the color of the paint marking on the springs - red for heavy duty - white for stock.
The shock (damper) is the same for both the standard strut and the heavy duty strut. The only difference is the spring. You can either purchase the heavy duty springs and install them in the standard strut - lower cost for parts/higher cost for labor - or buy the whole heavy duty strut already assembled and replace theentirestandard strut- higher cost for parts/lower cost for labor.