Context: I am talking about the panel right before Ran and Yus start dancing, where you can see a chamber group of string instruments. I am a music performance and education major in college, so I like to nerd out about music sometimes!
Interesting to see a viol, or fiddle a gamba, among other string instruments, such as violin, viola, and cello! The medieval viol is an upright bowed string instrument that you sit on your lap, much older than the modern violin that you may be used to. Usually, you don't see many arrangements of viol with other "classic" string instruments, since they both belong to different musical eras. Considering the violin was created in the Baroque era, early 16th century, and this strange instrumentation includes the viol, which was prominent during the Renaissance period, late 15th century, I'd say this webtoon takes place during the 1600s-1700s, if you applied real world logic.
It's also entirely possible that the artist just grabbed a random "string quartet" asset from Clip Studio Paint and used that without much thought lol
Anyway, this has been JUST a game theory! A game theory! Thanks for reading.