What's the difference between a TSO Authorization and a letter of TSO design approval? FAA can issue two types of approval for a TSO part application: TSO Authorization and letter of TSO design approval. What's the difference?

Does it relate to the approval being design-only or production only?

Also, do other Airworthiness Authorities have a similar difference?

According to 14 CFR 21 Subpart O §21.601:


  (2) A TSO authorization is an FAA design and production approval
  issued to the manufacturer of an article that has been found to meet a
  specific TSO;
  
  (3) A letter of TSO design approval is an FAA design approval for an
  article that has been found to meet a specific TSO in accordance with
  the procedures of § 21.621;
  
  (4) An article manufactured under a TSO authorization, an FAA letter
  of acceptance as described in § 21.613(b), or an article manufactured
  under a letter of TSO design approval described in § 21.621 is an
  approved article for the purpose of meeting the regulations of this
  chapter that require the article to be approved;
I don't know much about this, but it looks like a TSO Authorization is for domestic (US) manufacturers, and a letter of TSO design approval is for foreign manufacturers who want to import their products into the US. See 21.621:


  §21.621   Issue of letters of TSO design approval: Import articles.
  
  (a) The FAA may issue a letter of TSO design approval for an article—
  
  (1) Designed and manufactured in a foreign country or jurisdiction
  subject to the export provisions of an agreement with the United
  States for the acceptance of these articles for import; and
  
  (2) For import into the United States if— [...]


And the FAA's page on it says:


  What is a letter of design approval?
  An FAA design approval for an article manufactured outside the United States that meets a
  specific TSO.


I have no idea about other aviation authorities, but I assume there are similar regulations in other countries because the same general issue exists everywhere: how do you ensure that both manufactured and imported aviation components meet all required standards?