Defcon 19 Pyramid Cipher

Found at the Defcon 19 Rotunda, between the Registration Desk & the Swag booth. I saw tweets this morning that a foot print sticker was added on Sunday.

1 D
4 E
11 F
16 C
24 O (foot print)
29 N
33 A
35 W
39 G
45 B
47 U
56 Y
58 K
62 R
64 I
69 S
73 V
78 X
80 P
84 L
89 Q
94 M
99 H
104 T
? Z