It is a tough one. I'll give it a stab and say 12L12. Compares favorably with t he only copy I could find on-line at Siegel and is reproduced below as best as Siegel can allow. Also the faint blur going down the right side of the stamp. Also the small blur at about 2 o'clock in the O of ONE. The blurry bottom line and faint, almost broken top line.
Note the dot at the lower left of the lower left plume.