It sounds like your done with the thread and have gotten your answers. I apologize but in wanting to get my $.02 in I did not read them all because I found that I disagreed strongly with quite a few and didn't want to get balled up in a SOTW bar brawl. So, here's my take:
Like you I am a devotee of Lester Young and have transcribed and bought the transcriptions of most of his recorded solos. These were helpful but not conclusive. Since we deal in vintage horns there is a constant stream of old Conns and Bueschers flowing through here and often they include a mouthpiece in the case. These findings are mostly rubbish but every so often there is a gem. The result is that my collection of early mouthpieces, mostly untouched by the re-facer, has grown to respectable size if that is not an oxymoron on it's face.
In my quest I used a couple of assumptions about Lester based on his biographies most of which I have read. Some of his mouthpiece choices were driven by his health which was never very good and from his late thirties on was abysmal. His teeth were particularly bad and probably caused him some pain when playing and must have affected his embouchure to some extent. Therefore I believe that he tended to play mouthpieces with small tip openings for comfort and because it was the style of the day.
So, cutting to the chase, I have found that tip openings of around .070" (4) but no more than .085 (5*) and having fairly large chambers, coupled with disciplined adherence to copying his phrasing style, provide the best approximation of the Prez sound on the Conn New Wonder, Lester's horn of choice for most of his career. There are numerous photos of him playing of a Link ToneMaster but also on a black HR piece with an Ebolin sort of shape. I'm sorry that I can't be more precise about what piece he played but other than the major specs on the mouthpiece, it's really irrelevant since we've pretty well proven, at great personal expense to a lot of people, that duplicating a famous player's setup absolutely does not duplicate that player's sound. Therefore we have to fiddle with the setup until we get the sound we want using our chops.
Fortunately the mouthpieces I am talking about are not expensive and carry Buescher or King stencils. A Brilhardt Ebolin in 4* or 5 with a 2M or 2H Rico Jazz Select reed does a fine job but also one of the old white Buescher pieces, untouched of course, does just as well. There are many more that do the job with of course nuances of tone color that let you tailor your sound toward the Prez ideal. But, keep the tip opening small and don't overblow the piece and you may be surprised with the outcome. It all works better if you record yourself and listen as your own judge.
Your Pal,
Siggy