IIRC, the Monarch 10ee was a high-end tool room lathe, not a production lathe, and, as far as I know, they have not been manufactured in over 40 years or so. However, they are a primo machine and if in good condition demand a good price. Again, and don't quote me here, I believe they were only built in one size ... about a 12" swing and 20"-24" BTC.
The 10ee was ahead of its time somewhat in that it used a variable speed drive that was built integral to the machine. Unfortunately, that very fine feature is now its weak spot as the original variable speed drive was a complex system of vacuum tube technology that is now long obsolete. Due to the integral build of the variable speed drive into the lathe, it is not just a simple matter to replace the drive with a new design. Not impossible, but not easy and not cheap.
If in good mechanical and good running condition, a 10ee is a highly prized find.