I had to: (or more accurately my instructor had to do for me
1. Get a full background check
2. Show my competition itinerary and accomplishments
3. Explain how long I'd been training
4: How long I'd been teaching
5: was supposed to get a kata certification through IJF (though I got an exemption)
And even after that there was some pushback due to the fact I'd only officially done it like 8ish years.
My instructor had to write them a formal letter explaining that in 20 years of teaching He'd never promoted to black belt and that his standards were very high.
The thing that tipped the scales into my favour were
1. He had luckily had competition records of me competing and placing in black belt divisions for abut 4 years
2. His father also sent in a letter
He was an Olympic alternate and he was in super tight and very well known In NANKA (Southern California federation of IJF)
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My BJJ belts, every last one, I just got told I was now that belt, luckily for me it was from instructors that were very legit.