First I understand Rule 144 - and why a opinion letter is needed.
Beware of Attorneys bearing Rule 144 Opinion Letters.
Rule 144 allows public resale of restricted and control securities - but certain conditions must be met - that is why OWCP used a disbarred attorney to write the opinion letter.
Here is a example from a SEC press release:
"The Securities and Exchange Commission today charged a California-based lawyer who has been fraudulently churning out baseless legal opinion letters for penny stocks through his website without researching and evaluating the individual stock offerings."
Since there wasn't a registration statement filed for the stock gifted to Friedland - they were restricted and not free trading.
In doing real DD you need to understand securities law and there isn't a link to satisfy noobie investors but if there is a 144 Opinion Letter it is to remove the restrictive legend on the stock certificate.
Transfer agents require the company that issues the securities (stock) to provide a Opinion letter usually from the company Attorney - to state that the resale is exempt from registration so that the securities will be immediately free trading.