If you’re looking for the first three letters that were listed under January 1942, their dates were corrected and they were moved to February 1942.
If you remember back to when you used to write checks for things like your landline phone, you know how easy it is to keep writing the previous month or year a ways into the next one. I think that’s what happened with these few letters, and I’m sure I’ll run across it again. Walt gets all the way to the end of January and keeps writing, correctly changing the day but keeping the previous month.
Meanwhile, I’ll finish January before moving on to the rest of the February letters, trying to catch any mistakes like that in advance.