Flatwound strings were the standard type of string found on electric basses in beginning.
Flat metal strings had been developed for the upright bass for some years as a replacement for the “cat gut” strings originally employed on bass violins. (You can just imagine how upset the animal rights people would have been in those days.)
These types of strings provided better tone and were more responsive on the upright bass.
These strings are formed with a center core wire. Then the next layer is usually a round wire rapping, similar to a piano string.
Finally, the outside layer is thin metal shaving, wrapped around the core layers. This string has a very nice sound for the first couple of hours. After that, the sound which results is the sound that remains.
Generally, there are not a large number of overtones associated with these types of strings.
The “flatwound” type of string is the string of vintage bass playing and sounds. These were used largely in the 60’s and early 70’s music.
The best sound, in my opinion, is in the first three hours of usage. After that, they generally have a very flat sound, where most of the mids and highs have rolled off.