The general difference between SL and SLX tubes is that there are additional reinforcing ridges added to SLX tubes at the bottom bracket area, to stiffen things up a bit. The rIdges in the SLX seat tube also reinforce that tube where a front derailleur braze on is attached.
The first generation of SLX tubes has reinforcing ridges on the inside of the chainstays, where the tubes enter the bottom bracket shell. The chainstay ridges run lengthwise and are not spiraled, like the reinforcements on the seat tube and down tube (like those already existing on the steering tube).
Apparently the chainstay ridges were a stress raiser, and created some frame failures. Some harder used frames of original SLX developed stress cracks on the chainstays. The reinforcing ridges on the chainstays were eliminated on the "SLX New" tubes.
I was told directly of the frame failures of original SLX chainstays by framebuilder Roland Della Santa. Here is the SLX card from Columbus, and a section of a tube used to build my 2017 DS.
Columbus SLX Tubes card, with sample of tube
For the Fondriest, I do not know whether there are any differences in their SLX tubes compared to standard production SLX.