Regarding your question about the origin Ridvan messages sent annually, I will offer the following information for your interest.
“Ridvan messages: As early as 1923 Shoghi Effendi sent a letter of encouragement and greeting to the American national Bahá'í convention at Ridvan. Later it was his regular practice to write a Ridvan letter to the Bahá'ís of the world summarizing the progress of the Faith in the previous year and setting out general directions for the coming year. The Universal House of Justice has continued this practice. Other Bahá'í institutions, especially national spiritual assemblies, also sometimes issue Ridvan letters.” (From an article titled ‘Ridvan’ by John Walbridge and published in ‘Sacred Acts, Sacred Space, Sacred Time: Bahá'í Studies volume 1’, Oxford: George Ronald, 1995)
Although sending out Ridvan messages has been a practice, I have never gotten the impression that it was mandatory. Should others on this forum, or elsewhere, not provide answers to your latest query, and if having the answers related to those specific dates be of significant importance to you, I would suggest that you write to the Bahá'í World Center at email@example.com Beware though, and from my experience writing to the Research Department, getting an answer from the World Center can take several months and one must exercise patience. The individuals working there are immersed in their prioritized work which understandably must come first.
Thank you for answering my questions! I’m far from saying that sending out Ridvan messages is mandatory. I’m just curious why those particular years were left without usual letters from UHJ to the Baha’i World.