no letter = 1.0 (Japanese Release) 1.5 (North-American release) confirmed
A = 1.5 confirmed
B = 1.51 confirmed
C = 1.52 confirmed
D = 1.52 confirmed
E = 1.52 confirmed
F = 2.0 confirmed
G = 2.01 confirmed
H = 2.50 confirmed
I = 2.60 confirmed
J = 2.70 unconfirmed
K = 2.71 confirmed
L = 2.81 confirmed
M
N = 3.03 unconfirmed
O = 3.10 unconfirmed

So you'll want a K or lower to downgrade without GTA.

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