It is speculation based upon observation...Do you know why they skipped j?
Legibility reasons / easy to mistake for another with the script they were using?
What is fact is in my limited trends research this is a consistency with all makers but Danzig (they used distinctive i & j during 1916 & 1917, - 1915 they seem to have skipped j-block but too few examples to currently say for sure); that and in this case (Amberg/17) 7 observation for Amberg i block and 4 k-blocks but none for an identifiable j-block. This holds true with more examples i-k recorded in 1915; of course Amberg/18 ends in the h-block so that is the is the basis for my opinion.
***I doubt this will be taken as fact until someone else proves (as an overwhelming probability) it with more trends work, but I will say it is at least a probability at this point.