My daughter goes to a chabad school (nusach ari?) and has been taught in class that וּכְתַבְתָּ֛ם in the shema (end of first paragraph) is with a shvah na on the כְ. She also says it has the asterisk ontop of the chaf in her siddur.
However, I have only seen it in the siddurim (ashkenaz) with a shva nach (silent).
Is there a difference with this pronunciation between the different nusachot?