Update: After doing some testing and digging through the CK files, I found that damage boosting Legendary effects seem to only affect the physical damage of the Mr. Handy Buzz Blade, and not the electrical damage from modding it. This probably is how similar melee weapons work, just something I found interesting.
I also found the Leveled Lists for Legendary weapons, and I found that, despite the Assaultron Blade having a Legendary attach point, it doesn’t show up in them. And I can’t recall ever receiving a Legendary Assaultron Blade before. So I added both it and the MHBB to the Legendary melee list. ABs will start showing up at level 17 and MHBBs will start showing up at level 19.
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A simple esl flagged esp that adds the Legendary attach point to the MHBB. Note that I haven’t done much testing, but at least damage increase mods should work fine (eg Furious and Assassin’s). Not entirely sure if they’ll be added to the Legendary pool (I’m currently working on another mod so if they aren’t I’ll come back to see about adding that later), but if you’re using a mod that lets you attach/swap legendary mods then it should work with those.