You are correct that in KJV, verse 21 only uses the word "bowels" alone instead of "womb miscarry" like in this NIV translation. But here is an ESV translation that also says it will make the "womb swell" indicating that the "curse" is supposed to affect the woman's womb.
It's clear from multiple translations that it's not just in the "belly" or "abdomen," but the effects of the curse are supposed to affect the potentially unfaithful wife's "womb." I'm sure other of the many translations of the Bible also use similar terms to describe this prescribed abortion. If an unfaithful women has a curse put on her to affect her womb, it's not going to be a good thing. This is the rules of how to potentially initiate an abortion if the man claims he thinks his wife is unfaithful, even if there's no witnesses to substantiate such claims.
You are correct that in KJV, verse 21 only uses the word "bowels" alone instead of "womb miscarry" like in this NIV translation. But here is an ESV translation that also says it will make the "womb swell" indicating that the "curse" is supposed to affect the woman's womb.
https://www.biblegateway.com/passage/?search=Numbers%205%3A11-22&version=ESV
The NRSV translation also described the effect of the "curse" as "uterus drop, your womb discharge."
https://www.biblegateway.com/passage/?search=Numbers%205%3A11-22&version=NRSV
It's clear from multiple translations that it's not just in the "belly" or "abdomen," but the effects of the curse are supposed to affect the potentially unfaithful wife's "womb." I'm sure other of the many translations of the Bible also use similar terms to describe this prescribed abortion. If an unfaithful women has a curse put on her to affect her womb, it's not going to be a good thing. This is the rules of how to potentially initiate an abortion if the man claims he thinks his wife is unfaithful, even if there's no witnesses to substantiate such claims.