That's the same chart that's in my MP Chassis book.
Yeah, .850 is small, stock spec for this application is .880 (FSM) or .900 (FBBO & other sources).
It doesn't act as "lively" (lifting the nose) as a drag car when driving, and seems quite firm when you bounce-test it.
Would have never guessed they weren't stock had I not measured them.
But if they aren't up to the task of 4,000 miles of street use, that could maybe explain the LF adjustment being used up.
And the 456/457 #s don't correlate to anything I've seen so far (the chart or the broadcast sheet).
I don't have an original parts book to check.
Guess it's a new pair of .920 bars for the thing, to match the XHD rear springs...