My buddy and I were considering ordering some of this M855S. He emailed me a bunch of AR15.com threads about the round.
IIRC the consensus was that the M855 penetrator is difficult to manufacture in the exact center of the round. For that reason, the M855 is somewhat less accurate than M193.
Someone somewhere (a .gov type I'm sure) likely wanted this round and ordered a bunch of it, possibly backing out of the order. I think that was the best guess as to why IMI produced this round.
But the feeling was that the M855S would be more accurate than the M855 due to the more consistent core construction, yet it would retain other characteristics (minus the pentrator qualities) of M855.
Do a search here on the round and you should turn up some information.
FWIW, I have had two good experiences with Botach, and they were both higher end purchases ($$). But my rule when dealing with them is to order by phone and confirm they have the item in stock. Albi, the gentleman who I dealt with, said they had this Izzy ammo stacked up to the ceiling, so it should like they have a lot left.
I ordered some of the IMI M193 (same as what Winchester sold as Q3131A) last week and it arrived yesterday. I am very happy the ammo and how the order was handled. At $0.16 per round I would definitely recommend the M193 to others.
HTH
EDITED to add that it's my understanding that the M855S is called "match" only inasmuch as it lacks the M855's pentrator core (and should be more accurate as a result). But I do not believe that this is made to the high standards of other true "match" rounds. I.e., it's still mass produced field grade ammo.