Hi Christof,
I had a Burnham pen recently that had a J on the nib as well. Completely different font though.
Have a look here: http://bit.ly/1lrXObN.
Burnhams used the letter J for its calligraphy nibs, but I really don't know why and if the J on Tom's and your pen is an indication for a calligraphy nib as well.
Best regards from The Netherlands,
Allard Borst