My guess is that -30- was first chosen as the typographic representation of the end of a floral ornament used to signify the end of a text. For example, check

My guess is that -30- was first chosen as the typographic representation of the end of a floral ornament used to signify the end of a text. For example, check these: https://www.freepik.com/free-photos-vectors/text-end-dividers https://www.freepik.com/free-photos-vectors/text-end-divider... Several of the ornament endings can be stylized as ---3O-