Dear Avira,
My mistake. Iraqi Jews was NCB replying to AAQ.
It’s in Sefer Chazon Ish.
https://hebrewbooks.org/pdfpager.aspx?req=14334&st=&pgnum=118
The sugya isn’t anan sahadi. It is how to ascertain that food is prepared in a kosher manner. Rav Belsky had higher standards than Rav Moshe in general. Accordingly, it becomes a kulah. I’m sure you know that Rav Belsky was meticulous with his halachic approach. Your not quoting the Igros correctly. But even like you say, it doesn’t help your argument. Rav Moshe is very clear that it is muttar without any kula.
I don’t understand how this can be a machlokes what Rav Moshe himself held. He had chalav hacompanies in his house. There are many living witnesses to this.