Why are you asking for an address or sending an incomplete letter on a prequal at all?
A prequal is not an application so it can't be incomplete.
It's a request for an if/then guestimate on the loan amount the borrower should likely qualify for.
If it's truly a prequal, it's done when you send them the letter saying 'if all checks out, you should qualify for $x amount loan.'
Neither Reg C or B come into play on a prequal so I don't understand why you're sending letters as you outline above unless your marketing to get an application?