There's a Butter Krust bakery in San Antonio that still exists. The company may be a descendant of the original Butter Krust. See this link:
http://www.lib.utexas.edu/taro/utsa/...tsa-00071.html
After operating for 100 years, and marketing the Butter Krust brand, advertising signs are pretty easy to find. The one you found is probably not early 1900's. Just my opinion from looking at old signs.