Well in terms of how it plays, here's the funny part.
In recent memory, I played it *once* on a gig. Ironically, the gig paid *really* well (tv commercial shoot. Gotta love resids). Of course the 4th valve just sat there unused.... But the horn looked good! Most of it was just us miming. Or rather, standing around in our fetid sailors outfits waiting to mime.

They recorded some "wild" tracks live, but IIRC, they weren't used. (The band did sound good, but I think the techs were just trying to look busy and recorded us in the down time during the shoot....But I digress.)
It may very well play great. It seemed fine to me at the time. Too little playing to tell and I had trombone face at the time. As to it being rare, my dad didn't make lengthy notes on his correspondence with Leblanc years back, but the gist of it is that Holton made few, of these and they might have been a special order of some kind. I searched around some more in my file cabinet and found correspondence I eluded to before. Sorry for not posting the info first.

Here 'tis
On a piece of orange-ie/yellow Leblanc note paper sent back to my dad:
"Low Pitch Mammoth Holtanphone, Finish D - Silver w/gold bell. Made 12/3/23"
(the bell may well have been replated. I don't see the gold finish)
My dad's handwritting on that note: "Four valve can't be remembered. Very few (exceedingly few) four valve horns were made. Mine is prob the only one!"
There is also another note, by his hand, with "PAS 6093 Paramount Records. Jack Daniels Original (can't read) in concert. I googled this and it got a hit. Maybe this horn was used way back to record, but according to this web page
http://www.bsnpubs.com/dot/paramount.html
the recording was "PAS-6093 - Jack Daniels Silver Cornet Band - Jack Daniels Silver Cornet Band [1974] " which doesn't make sense as he purchased the horn in the early - mid 50's.
The thot plickens......
OT