I never entertained any doubt as to "too full of lines under the words to be made public" meaning a certain handling of *double entendre* which could convey information about private matters. But this thread opens a new perspective, simpler, and perhaps more in keeping with Lydia's character.;)

However, the fact that those lines could not "be made public" might infer that indeed there were double meanings to certain words or sentences, but as Lydia certainly was not the cleverest of letter-writers, their *hidden* message was too blatanly obvious for them to be shown to the family. Even Kitty was smart enough to see that.