prepositions - Meaning of ON at the end of a sentence

There is ON at the end of the following sentence.

Remaining aloof was no longer safe under the restored democracy of 403 on.

403 is the year.

I think without it the sentence makes sense enough. I know there is no usual sentence that ends with a preposition and there is no object of it. If I understand it as an adverb I can't find any verb that matches it.

Does it have any special meaning? If so what does it?

Answer

It is the adverb (more usually "onwards" or "onward") indicating the direction in time from that specified in the sentence (403) or context (the restoration of [Athenian] democracy).

https://dictionary.cambridge.org/dictionary/english/onwards

The sentence would be understood with "on" omitted, but the usage implies a continuing danger here, as opposed to a brief period during the restoration year/s.

https://sentence.yourdictionary.com/onwards