The Seeburg M100A was the first in it's kind that allowed you to insert and play 100 selection (50 records with 2 sides). Other systems at that time only allowed you to insert around 20~30 records.
This was all possible due the rail system that Seeburg used for this jukebox, since the records were not stacked horizontally but vertically it allowed you to use more space then ever before.
In the beginning of it's release in 1948 you could only use the old standard of 78rpm records that were made out of shellac. A year later Seeburg came out with the 45rpm version and provided upgrade kits for the current owners of the devices by simply replacing the whole record tray or installing a kit that converts the depth of the records and the record needle.