in Latin i and j are interchangeable (look at any Roman inscription) - when writing numbers in roman notation the tradition was to use J as the last of a sequence of 'i's both because the tail was easier to write but also the final j was a must clearer delimiter.
Meanwhile as English developed 'J' was adopted to signify another sound 'dw' (giving rise to disputes in prononciation of Latin between English + Continental singers) but the same tradition of using 'j' as the last symbol of a sequence of 'i's continued - the double i at end indicated the stress