Impossible, from what I know at least. From my knowledge sportline doeesn't have an SD6 version, only SD5. In addition to that, when looking at the SKU under the price on HO website, it says ICS-02, which refers to the metal SD6 version, as you can see here on ICS website.
http://www.icsbb.com/product/ics-02-mx5-sd6/
That being said, looking a little bit more. I saw the same gun 150$ more here...
http://www.herooutdoors.com/ics-bt5-...6-airsoft-gun/
Really getting me confused lol. Either the most amazing deal i've ever seen or something's wrong.
I emailed them about it, haven't received an answer yet