A lucky hit after starting the Formula BOINC challenge.
416Ed4Gqcz8tpVzmSFSdVynvgyKP found a megaprime for base R684.
The prime 39*684^519468-1 has 1.472.723 digits and entered the TOP5000 in Chris Caldwell's The Largest Known Primes Database. With this find it also has proven the base!
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