First, don't freak out. I was also freaking out over this (assuming you read my previous thread 🙂) and it ended up being not a big deal.
IIRC we contacted the company who provided the J1 to my husband, they did not keep records longer than a few years. We kept that email exchange for evidence.
We also submitted a couple FOIAs to try to track down records of the J1, we recieved replies that they did not find any records. We kept that exchange for evidence as well.
At some stage (maybe AOS, honestly can't remember), we included all that evidence with an explanation letter detailing everything we remembered about the J1. Basically just the year it was given and used and the agency it came from.
No one ever asked us about the J1 at any stage and it was never a problem.
I could be wrong but I think the only thing they are looking for is related to whether or not the J1 had the home residency requirement and if so, that it was fulfilled or could be waived. Was your fiance in their home country for 2 years cumulative time after the J1 was done through when you applied for K-1? Because if so, I think you're fine. If you dont lie/withhold info, there is no available evidence of your J1, and you prove you did your due diligence to search for it, what more can you do? The chance of this messing with your K-1 is incredibly low IMHO.