Someone with more knowledge hopefully will expand on this.
The more important distinction than manufacture date is the drive type. Really old Monarchs have a hydraulic drive (rare), most older ones have a motor-generator drive and can be quickly distinguished by the round dial for the feed rate selector. Later models have what's usually referred to as Works In a Drawer (WAID) that uses large vacuum tubes to control the motor. The feed rate selector panel on these lathes are square. There are even more modern drive types, but these tend to be somewhat rare.
The '41 is very probably a MG drive. If all the parts are present, these can often be restored without dropping loads of cash. These tend to attract people who believe that it would be simpler to replace the motor with a modern motor, which can turn out to be a bad idea in terms of being more expensive and difficult than it would appear. Original equipment is usually the better approach.
Lots of people get scared off by the massive and expensive vacuum tubes in the WAID model. However, if you do some searching on-line you will discover that the $1K original tube can be replaced with something considerably less expensive that works well.
The feed rate on the WAID models can be set much lower than the MG ones and consequently give a finer finish. The MG ones have more potential for a simple conversion to cut metric threads. WAID drive models can be run on 2-phase power (except for the coolant pump if so equipped). MG models need 3-phase.
All these lathes weigh in the range of 3500 lbs. Moving one into a basement can be quite the challenge.
Walt
