Observation: Both the letter to Pythocles and the letter to Herodotus start out with a specific detailed greeting explaining the purpose of the letter and saying that these people are being provided a summary of a part of Epicurus' philosophy.
The letter to Menoeceus, in contrast, does not start the same way. It does not identify the reason that prompted the letter, state the purpose of the letter, or refer to it being a summary of the principles of ethics.
The second paragraph does imply that what follows is in explanation of the things Epicurus "used unceasingly to commend to you..." but this is after an introductory paragraph that seems to be floating out of context. Is something missing, or was there something about the rationale of the letter to Menoeceus, or about Menoecus himself ("that I used to commend to you"), that is relevant for us to keep in mind?
This is in contrast to the letter to Herodotus:
And the letter to Pythocles in particular mentions this context, and even refers to the letter to Herodotus - but does not refer to the letter to Menoeceus: