Mike, you have been discovering some real gems that were never recorded in stereo. I was fortunate to be able to hear Munch conduct the BSO live, in the mid-60's when I was in college. I got to Boston in the fall of 1963 when Leinsdorf had taken over the BSO from Munch. However, Munch still did some guest conducting of the BSO and I was lucky enough to hear him conduct the Saint-Saens Organ Symphony with Berj Zamkochian playing the organ, the same artists who had recorded LSC-2341 a few years earlier.
One set of mono recording that you can often find in the $1 bins are the London in the US (or Decca in the UK) classical recordings done from mid 1954 to mid 1958, when Decca was starting to record in stereo, but had not yet started releasing stereo records (which started in 1958). What many do not know is that Decca would normally have two different teams recording the albums - one for stereo and one for mono. And the mono team usually got the A team - Kenneth 'Wilkie' Wilkerson was the A team engineer - since mono is where Decca was making is money. What I learned in my interviews with the old Decca engineers was that not only did they have two teams, they had, in the case of Kingsway Hall, at least, two completely different recording studios. There were two sets of mikes, two different tape recording systems, and completely different edits for recordings themselves. So, for example, in the famous Espana album (with Argenta conducting the LSO) Wilkie did the mono album using a different choice of edits and takes than the more famous and much more pricey stereo album. I think I paid well over $100 for my stereo Decca original (now going for closer to $500) while I paid less than $5 for my London original mono album.
Good luck continuing your mono hunt.
PS. Another great series of mono albums are the Westminster chamber music albums done in the '50's, many with the great Barylli Quartet and Vienna Konzerthaus Quartet, two of three string quartets who were members of the Vienna Philharmonic (the other was the Weller Quartet who recorded for Decca).
Larry