chamelion - my take is, they don't know, for sure. But it does seem as tho even with that batch of material they had, only a few slides actually developed any cracking.
Further thought is - carry on with practice sessions when needed and monitor your slide, plus and perhaps most important of all, I see no way that a crack where it seems to occur would ever render the gun inoperable in extremis.
That SN region is very arbitrary probably and spread also quite large ........ so, be confident that a) - if a crack should appear it will be taken care of by Bros R and b) - your gun will not suddenly stop working should one occur, if at all. That is only my own opinion on it.