Thanks to Tom Y for letting me review this instrument!
The 'three-cornered hat' designation is my own. I don’t know what name Leblanc used when they marketed this clarinet. Would it have been an early Symphony model? See the later model below which is called an LL model.
Bore LH joint top: 14.7mm
Bore LH joint at bottom: 14.7mm
Intonation summary: I play too flat to be able to use the 65.8mm barrel. Using my 64.2mm barrel I was almost in tune. My impression is that this would have excellent intonation for pre-poly-cylindrical bore clarinets. The low A was only 10 cents sharp.
Key work quality: Top rate. Note that this has the independent posts for the right-pinkie Ab/Eb key.
This clarinet is most appropriate for: anyone, including serious students and advanced players.
Condition issues noted: None.
The three-cornered hat model may be the LL Model.
This picture was from an eBay listing which had this information:
Very rare professional Leblanc Plateau (closed hole) "LL" model wood clarinet. Serial #51018.
Now, plateau clarinets are fairly common, however, I've NEVER seen a professional model. The "LL" was Leblanc's top-of-the-line. I've called four vintage woodwind dealers and all of them said they have never bought, sold, or even seen a pro Leblanc plateau. That being said, I have had a difficult time finding out much information on them. The music store I used to work in for in Nashville has a Leblanc catalog from 1996 and they retailed for $3800 back then. This means a music store would have had to pay around $1650 for one. I've called Conn/Selmer (which bought out Leblanc) and they don't have any records of them anymore but said they were last offered around 2005/2006. There you go…that's all the information I know.