I think the "H" stands for high speed (5000 rpm - I believe)., but I'm pretty sure that on a SL2-B the B stands for " barstock ".
Which I think means it has a bigger spindle bore than the H series.
I'm not so sure about the quality and dependability of the SL2-H, but I wouldn't think it would be any lesser quality than any other Mori.
I can tell you we have a 1988 Mori SL-20 which most likely replaced your SL2-H.
We put about 1000 hrs a year on it and have never had any problems with it since buying it in 2006.
Bottom line, most people that have a Mori lathe love them and think they're the best, I do.