Charles Ellicott Commentary Numbers 8:16

"For they are wholly given unto me from among the children of Israel; instead of all that openeth the womb, even the first-born of all the children of Israel, have I taken them unto me." — Numbers 8:16 (ASV)

Instead of such as open every womb, even instead of the firstborn of all the children of Israel. —It is difficult to determine whether the second clause is to be regarded as an exact equivalent, or as a limitation of the first. If an exact equivalent, a different meaning must be assigned to the firstborn from that which it commonly bears in the Pentateuch, where it appears to be restricted to the firstborn son on the father’s side .