Yes could be option for heated seats. They are deliberately different so you can't accidentally get them mixed up. Don't know anyone in the UK with this option but it's probably common across same-era Mazdas if you wanted to go hunting. Would need a fuse adding in the fuse box too. Also where would the switch would go - maybe there is a wire behind one of the blanks on the dashboard?
This post:
https://www.club323f.com/forum/viewtopi ... 18&t=51597talks about heated seats, and someone in the Netherlands says their 323 has them. But also mentions the wires might be used for an auto box