It's weird nothing in the Stackexchange answer makes the point about the letters not being separate. In Latin (at least during the Middle Ages) J and I were use

It's weird nothing in the Stackexchange answer makes the point about the letters not being separate. In Latin (at least during the Middle Ages) J and I were used interchangeably. It was simply a matter of style or, at times, the random whim of the scribe. A person's name might start with a J and then in the same document be an I. As for numerals, tacking a J (or really what often just looked like an I with a longer straight descender) was common in late medieval Gothic scripts. Forgery doesn't make sense, yeah. You gave some reasons, but also western medieval and early modern conceptions of forgery, for instance, were very different than our modern one. Ease of reading and clarity makes sense. Nothing in the answer (or anywhere in this thread) distinguishes "when". It don't doubt that in the nineteenth and early twentieth centuries the "j" was used in this way, but that might not have always been the case.