Alphabet {a, b, c, d} with 3 letters used

* 24 possible keys (= (4 choose 3) * 3!)

1 2 3

a b c
a c b
b a c
b c a
c a b
c b a

a b d
a d b
b a d
b d a
d a b
d b a

b c d
b d c
c b d
c d b
d b c
d c b

a c d
a d c
c a d
c d a
d a c
d c a

* Set 1 = a => 6 possible keys (= (3 choose 2) * 2!)

2 3
b c
c b

b d
d b

c d
d c