From Protype to Production model nunmbering

I was just curious because I was not aware of S-300 or S-400 and wondered why they didn't keep the S-300 or S-400 name instead of the S-500? There is F-100, G-200 and then S-500. Did the S-300 or S-400 go into production and were discontinued.. so it explains they were dropped and ended up with S-500? Is the S-500 combination of the 2 models? Or did they find there were models out there with the S-300 and S-400 names already? Thanks you in advance!!!

There are a few S-300 proto's, you might check the gallery.
I don't think there were any SC-300's made , however decals were made ( I have one)
so there's another to wonder about.