It's possible that the DM-504 set is a 78 RPM set. 78s are heavier and thicker. The groove size is also much, much larger than an LP. If the set is packed in a book like package, with paper sleeves attached to the binding, it's almost certainly a 78 set. (Although, I have seen one LP set from the very early fifties that is packed this way.) If this is a 78 set, be careful--the records were fragile, and should NOT be cleaned with an alcohol based cleaner. You might save such a set to play with--there are many people who still play 78s.
As for the stereo/mono question, it's actually easy to tell with many records recorded in the earlier days of stereo--the earliest stereo records would advertise the fact on the record label and the sleeve. But, some companies (like RCA) would have a fancy name for their mono records, and a promise that the record would sound good on your phonograph today, and even better on a stereo phonograph. These mono records might have been better than stereo--in fact, they often were (if nothing else, it took a while to work out the stereo bugs.) But, they are still mono records.
In time, if you have enough mono records, you might look into getting a setup where you can have a cartridge just for mono LPs. The sound is supposed to be much better--although it requires cartridge or stylus swapping, or else a second turntable.
Finally, the "microgroove" label is just talking about the long playing record technology. The LP groove is much smaller than a 78 RPM record, and the groove was called "microgroove."
As for the stereo/mono question, it's actually easy to tell with many records recorded in the earlier days of stereo--the earliest stereo records would advertise the fact on the record label and the sleeve. But, some companies (like RCA) would have a fancy name for their mono records, and a promise that the record would sound good on your phonograph today, and even better on a stereo phonograph. These mono records might have been better than stereo--in fact, they often were (if nothing else, it took a while to work out the stereo bugs.) But, they are still mono records.
In time, if you have enough mono records, you might look into getting a setup where you can have a cartridge just for mono LPs. The sound is supposed to be much better--although it requires cartridge or stylus swapping, or else a second turntable.
Finally, the "microgroove" label is just talking about the long playing record technology. The LP groove is much smaller than a 78 RPM record, and the groove was called "microgroove."