The Williamson-Haffner Engraving Company existed and was headquartered in Denver, Colorado from 1905 to 1915.
I have seen the message on the back of the post card several times. It must have been popular at the turn of the last century. "I don't know where I'm going, but I'm on my way." I am pretty sure that the word "Dearest" was scratched off much later by another "collector"; and the "mn365 20" looks like it was added after, too.
The post card was mailed 109 years ago today at 3:30 in the afternoon.