Mentioning this again because it's funny.
I have this long time crack conspiracy that due to being the one raised in the most noble household, Nobukatsu sometimes gets asked by his bros to write their formal letters for them so that it would sound super proper and fancy. The person in question then just later writes in the signature at the end.
Shitpost reasoning for it:
1) Of all the letters that I've seen, that "t"-shaped stroke in the 信 kanji is only ever done by the 3 of them, not even Nobubu himself does that. Obviously there's probably a ton of letters that are lost and maybe among those lost letters actually other people do it too, but still. The 3 brothers seem close and at least one account says that they do occasionally visit each other.
If my conspiracy is true, it could be that when they're together some important thing came up and bro got asked to write the letter under his brother's name.
2) There's some very odd letters where it looks like the letter-writer mistakenly wrote "Nobutada" as 信直 (usually read as Nobunao) and not 信忠. The only other name where I've seen -tada written as 直 is someone from Iga. Like, again, maybe actually it's normal and there's just no other surviving records of it yet. But if my conspiracy is true, it could be that -tada being written as 直 is a thing that Iga people likes to do.
The Kitabatake territory, of which Nobukatsu is the lord, kind of ate into a major chunk of Iga so maybe Nobukatsu slipped because he had been writing -tada with 直 so often. Plus the two occasions where this happened both involved Nobukatsu. Once was during a campaign where the bros are known to be out together, and second it's about how Nobukatsu's castle got kaboom'd and someone is being sent to investigate.