There's been some discussion about this on the Steel Guitar Forum -
http://bb.steelguitarforum.com/Other than the fact that the case was written on by somebody who couldn't spell "Bob Wills" (and maybe didn't feel confident about attempting "McAuliffe") it seems that this "1935 guitar" wasn't available until about 1940 (at least, the Gibson model, of which this is a copy, wasn't around in 1935).
Also, the letter of provenance says nothing more than that the guy who sold it to the guy he bought it off said it came from the McAuliffe family (and 2 un-named musicians said yes, it looks like one of Leon's).
Nobody has seen a picture of him ever playing one of these guitars.
The "It's A Beautiful Day" album the model is squatting on might be collectible though
