I said:
>The incidence of 1's in the second place from the right (excluding
>p=2) is 16/(16+21)=43.2%;

This indicates the degree to which Mp for p=1 mod 4 is more probably
prime than is p=3 mod 4.  Of Mp for p=3 and up, 
p mod 4  #   %
1       21  56.8 
3       16  43.2


>the incidence in the remaining interior bits is 172/358=48.0%

This last line was wrong. 
For p=3, the second bit from the right is what I've
been calling an exterior bit (most significant or least significant).
So the one bit for p=3 should not be subtracted from either the
one bit's count or the total bit positions counts for interior bits.
So the correct incidence in remaining interior bits is 173/359~=48.2%.

Credit for spotting this error goes to, who else, but George Woltman!

George asks, what about the bit second from the left?

In total, 26 0 bits vs only 12 1 bits. (0 26 of 38 times or ~68.4%;
26/12=2.16666...)
Excluding p=2 and p= 3, to look only at interior bits, 25 0 bits vs 
only 11 1 bits; 0 25 of 36 times, ~69.4%; 25/11=2.2727...)

Now why would that be, that a 0 bit is more than twice as likely as a 1 bit? 

(Note that Mp#38, M6972593, has the less likely 1 bit in the second most
significant position; searching biased for these probabilities would have
delayed finding it.  But the 4 previous Mersenne primes all had a 0 bit
in that position.)


Ken

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