The Brilliant Corners we heard at ProggRob's and what I have at home is the mono version. (You're right, Lon. The LP is available from acousticsounds.com.) The stereo, I think, was after the fact pseudo-stereo. There's a lot of that floating around from that period. "BC" is from Riverside and recorded in a Manhattan studio, but here's a discography of Blue Notes, nearly all recorded at Rudy Van Gelder's studio, that distinguishes mono, stereo, and fake stereo:
https://londonjazzcollector.wordpress.com/record-labels-guide/labelography-2/blu...e-note-mono-stereo-and-pseudo-stereo/
From the same web site, I discovered this essay on Van Gelder's transition from mono to stereo. He was the go-to studio for both Blue Note and Prestige and therefore a really important figure in ’50s & ’60s jazz. I'm still trying to get my head around monitoring a stereo recording with only one speaker!
https://londonjazzcollector.wordpress.com/2014/07/24/guest-post-how-they-heard-i...t-blue-note-records-and-the-transition-from-mono-to-stereo/
There's also quite a story about recording "BC's" title tune, which is really difficult to play. After about 25 takes, producer Orin Keepnews thought there was something wrong with the bass mic. Turned out that Oscar Pettiford had completely lost it by then and was only pantomiming. At that point, they gave up and the performance on the record is a collection of splices. And they'd run out of time, so they came back later for "Bemsha Swing." Needless to say, Oscar wasn't invited back, nor was Ernie Royal. Clark Terry and Paul Chambers are hardly second stringers, though.
I just listened to an album of duets by bassist Christian McBride with a variety of artists. The depth and control on his bass is breathtaking. I can visualize how and where he's striking the string. No one hearing this would ever disparage SET's bass performance again.