Forgive me if you feel this is not the best forum to ask this question on, but it seems the most apposite one out of those currently available.
That this arrangement was put forward in Jewish commentaries as a possibility, to explain why others might not have been able to decipher it, is noted in an article by French archaeologist Claremont-Ganneau, who references this source, although he does not give page numbers, so I have not been able to look into this further.
So there seems to be a precedent for the arrangement that Rembrandt used.
A couple of sources (on The National Gallery's website, for instance, and this article exploring Rembrandt's links with Jewish culture) state that he got one of the characters wrong. The latter source notes that he had originally painted a 'zayin' as the last letter, when it should be a 'nun sofit', but overpainted it.
There is more precision in Michael Zell's book, 'Reframing Rembrandt: Jews and the Christian Image in Seventeenth-century' in which he shows the x-ray evidence relates to a shift of the vertical descender from the right to the left (p 61). However, Alan Cohen argues that the shift may have been deliberate.
Is there a mistake, or isn't there?