4423/Historical Note

Historical Note on $4423$
The Mersenne number $M_{4423} = 2^{4423} - 1$ was demonstrated to be a Mersenne prime by on $3$ November $1961$, using an IBM 7090, along with $M_{4253}$, whose primality was similarly demonstrated in the same computer run.

These were the first titanic primes ever discovered.

It is a moot question which one was actually discovered first.