Excuse me but I answered your question ... maybe my English is limited or stupid.
Have a look on this:
Attachment 15932
How you see the handwritten I looks like the printed J (depending from handwriting and/or print).
The usual form of an "printed I" was an printed I
Attachment 15934.
Of course in your example was used an printed J instead of an printed I and you asked me for an explanation and this question I answered:
"There are a lot of possibilities --- the correct stencil wasn't by the hand or used by others, an unskilled worker with problems in spelling (such peoples in Germany too), not enough time/ stress (but him was known that everbody understood the word because the handwritten I and the printed J have/had the same form) or... or... or..." ... or how we say in Germany: "Ausnahmen bestätigen die Regel." ("Exceptions confirm the rule.")
But I believe this is an academic discussion and not part of an aircraft/airwar forum. So I believe it's more useless...