But CBP fills out Canadian TN visas...some of them. Others may not be a TN visa, but an I-129 processed by USCIS. Mexican TN visas, they have to apply at an Embassy, so they're processed by State...
The main point being, it's a totally ***** up state of affairs.
Clearly you work at DHS, because you call alien numbers "A numbers." Every - every - immigrant I've talked to calls them "alien numbers," and thinks "A-number" is ridiculous when I tell them I'm not allowed to call it that.