Most supporters of GWB got this letter. They are produced by an "AUTO PEN". He didn't actually sit and write out each letter, which are all the same except for the name of the recipient. The auto pen is also used for the majority of white house christmas cards and other greetings.
GWB writes the letter and signs it 1 time and then a machine writes the copies. It is not "copied" like a photocopier, each letter is actually written by the machine, an exact duplicate of the original written by the president. A nice touch to "appear" more personal while still recognizing that the president is too busy to write out thousands of thank you letters to supporters.
RGW