Another possibility where GMI would be required under Reg B but not under Reg C would be a particular type of bridge loan. If the bridge loan is to fund the purchase of the primary dwelling, and secured by that dwelling , then GMI would be required under Reg B but not under Reg C since bridge loans are exempt under Reg C as "temporary financing (such as bridge or construction loans)."
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Not a legal opinion, just my personal opinion.
"A nickel isn't worth a dime today."- Yogi Berra