The letter "J" was the last letter to be added to the alphabet, and it was used interchangeably with the letter "I" so when you look at hallmarks which begin with the letter "I" you always have to try to figure out if the silversmith's name actually began with the letter "J" (until around 1850 when the "J" really seems to be firmly established). :::
The letter "J" is only about 400 years old so the tradition of using "I" and "J" interchangeably seems to have hung around for a couple more centuries:
"Both I and J were used interchangeably by scribes to express the sound of both the vowel and the consonant. It wasn’t until 1524 when Gian Giorgio Trissino, an Italian Renaissance grammarian known as the father of the letter J, made a clear distinction between the two sounds. Trissino’s contribution is important because once he distinguished the soft J sound, as in “jam” (probably a loan sound), he was able to identify the Greek “Iesus” a translation of the Hebrew “Yeshua,” as the Modern English “Jesus.” Thus the current phoneme for J was born. It always goes back to Jesus."