Cray,
I just received the 119 last night so this is an initial impression..
The knife itself is awesome! Hits the right mark on every level, weight, balance, ergo's, blade geometry. In a nut shell I love it! But more on that some other time after I've had it a bit.
To get right to your questions, the Boltaron sheath.
First impression is very positive. Excellent retention. The knife locks into the sheath without use of the retention strap. I held it upside down and shook it and the knife stayed in place. Great "detent" for lack of a better word. The retention strap then snaps snugly in place, holding the knife with no movement once in the sheath. Great retention but not overly difficult to release or deploy the knife.
As far as the back plate, I agree with the observation of one of the posters above. At first glance it does appear kind of "flimsy" but I suspect its a lot stronger then it looks. Once locked in place it does feel secure. As far as longevity and strength, only time will really tell but off the bat I do have confidence in it.
Compared to other stock sheaths I own (Ka-Bar / Becker, Cold Steel, Condor) it more then hold it's own and I'd say it's one of the better ones if not the best. I should add I don't own any custom of after market sheaths to compare it to, only other stock Kydex and nylon sheath's so take that for what it's worth.
Anyway, Like I said, this is an initial out of the box first impression and I'm sure I'll have more to add as time goes on but I hope you find it useful just the same.
All the best!