It's my understanding that companies that are required to file reports with the SEC aren't eligible for the Rule 504 exemption. I believe they may be able to use
Some reporting OTCQB companies use convertible debt and convert the debt at a steep discount to the market. If the debt instrument is over one year old, it's my understanding that the conversion shares are often issued as free-trading.
You are correct that shells (blank check companies) are prohibited from using the Rule 504 exemption and that many shells falsely claim they are development stage companies.