Livre d'Or

According to Wikipedia, that rhyme in contemporary English is really characteristic of the accents of the north and Midlands of England. So it's not surprising you wouldn't have come across it, for me it's the accent of roughly half the people I talk to, but you probably know a lot fewer people than I do with accents from regions of England. I think you're right that a visual rhyme is likely to be a trace of what would have been a rhyme in the prestige dialect of the time a text was written.