That's kind of odd...in the memo he says a bunch of things, but if you go to the Firearms, Weapons & Knives policy, it lists the following things as permitted:
All other legal gun parts and accessories, including, but not limited to, trigger guards, firing pins, barrels, stocks, butt plates, trigger assemblies, pistol grips, holsters and scopes. Listings for such items must clearly state the type of firearm for which the part or accessory is designed. If this information is not provided, the listing may be ended.