If I'm pretty sure I had money in Mt Gox but I'm not sure how much, how could I ever file a "proof of claim"? It's not like Mt Gox was sending depositors balance statements every month or something.
If you registered at MTGox before mid-2011 you can use the user database leak from around then to find your userID, which you can then use to search the trade records.
Download this file WARNING FILE CONTAINS VIRUS, DO NOT RUN EXECUTABLES INSIDE ZIP. To be safe, extract the files inside MtGox2014Leak.zip\backoffice\Exports __ONLY__: https://archive.org/details/MtGox2014Leak
The exports folder contains 2 zip files, one contains a list of transactions in csv format, the other contains the output of a sql query displaying all balances.
Go into your e-mail records and find any transaction id, it will look like a v1 UUID (example: 682b3d0a-67d7-4b62-8ed4-4e1d39d54a69)
Look for it inside the btc_xfer_report.csv. The UUID before it on the same line is your wallet.
Look for your wallet UUID inside the mtgox_balances file. If it's not there you either didn't have a wallet at the time of the dump, or your balance was 0.
https://archive.org/details/MtGox2014Leak
DO NOT USE THE EXECUTABLE IN /backoffice/Bin/TibanneBackOffice.zip - IT IS MALWARE
Unzip /backoffice/Exports/mtgox_balances.zip.
Your user ID is a UUID, and can be obtained from your Mt.Gox account creation email, from the account confirmation link (example, sanitized)
ht_ps://mtgox.com/signup/validate?ID=1111aaaa-bbbb-cccc-dddd-eeeeeeffffff&Code=1234567890123456
Search or grep for this UUID in the unzipped mtgox_balances file and you should have values for BTC and your primary currency. Divide these values by 100000000 to get actual amounts.
The 'stamp' column appears to be a timestamp of last update of these values, and the last date in this leak with significant activity is 2014-02-25.
Also, more info here: https://www.reddit.com/r/mtgoxinsolvency/comments/8t0lbq/lat...
"Further, due to the stay of the bankruptcy proceedings, MTGOX bankruptcy claims filling system (the “System”), which was being provided on its website, has been temporarily suspended. As in the bankruptcy proceedings, the Civil Rehabilitation Trustee is expected to accept the method of filing a proof of rehabilitation claim through the System in the Civil Rehabilitation Proceedings as well. Therefore, please take note of your user name and password registered on the System."
[1] https://www.mtgox.com/img/pdf/20180622_announcement_en.pdf