There are four versions of the Liberty Bell Forever stamps, all released in April 2007, which were printed by three different contract printing companies. The stamps from each printer have a distinctive font and letter size used to microprint the word "FOREVER" in a small area on the bell's mounting, just to the side of the Liberty bell.
Two versions of the Liberty Bell stamps (Scott #4125 and Scott #4128) were printed by Avery Dennison, with
large microprinting:

Scott #4125
Large microprinting, Bell 16mm wide, serpentine die cut 11 1/4 x 10 3/4 on 2 or 3 sides.
Scott #4128
Large microprinting, Bell 16m wide, serpentine die cut 8 on 2, 3 or 4 sides.
The two Avery versions with large microprinting can be distinguished by their different size "perforations" which are actually simulated perforations due to the different gauge die cuts used to make the seperation lines between the stamps.
The third version of the stamp (Scott #4126) was printed by Ashton-Potter with the word "FOREVER" in
small microprinting:

Scott #4126
Small microprinting, Bell 16mm wide, serpentine die cut 11 1/4 x 10 3/4 on 2 or 3 sides.
The final version of the stamp (Scott #4127), was printed by the Banknote Corporation of America for Sennett Security Products, and has the word "FOREVER" in
medium microprinting:

Scott #4127
Medium microprinting, Bell 15mm wide, serpentine die cut 11 1/4 x 10 3/4 on 2 or 3 sides.
Unfortunately your picture is too small for me to tell which version of the stamp you have, but you should be able to identify it now.
Sorry it took "FOREVER" to answer your question.
