My guess at the stamp is, judging by the corner decoration and the curve at the bottom of Jeffersons bust (get yer minds out of the gutter!) that this is a #11. Colour is difficult with this scan, possibly an 11c greenish blue.
Here are my Jeffersons, an 11 and two 12s.

Look at the bottom curve of the 11 on the left and see how it is steeper than the curves on the 12s. Also the corner decorations on the 11 are a little less elaborate.
The 12 also has a fine line outside the frame at the corners. Check and see if it is there on yours.
The 12 on the right is a 12c bluish green. Might be close to yours, again hard to tell on the scan.
So that's my guess 11 or 11c
thanks