I found them for sale, but I can't figure out where they came from. On the back of the green plate, it says GB in marker, so maybe United Kingdom, but I'm not sure.
I saw these license plates for sale, and I'm not sure where they are from. It looks like it's from Spain, but I'm not sure because of the color.
The Peel P50 has a yellow license plate with black text, like a Kei car, but since its engine is only 49cc by default, it falls into the microcar category (<= 49cc). Therefore, its license plate should be light blue with blue text.
source:
Vehicle registration plates of Japan - Wikipedia
港区ホームページ／原動機付自転車等オリジナルナンバープレートの交付について
the picture is from u/TheRacingMonkey (If it bothers you, I can delete it)