The provisions of The Plumbing License Law of 1955 shall not apply to:
A "backflow preventer," as used herein, means any permanent mechanical device, or combination of permanent mechanical devices, of whatever material, which, after installation acts to prevent a reversal of the normal directional flow of potable water within the piping system in which it is installed, and shall include, but not be limited to, metal checkvalves and airgaps, either naturally or artificially created. Provided, further, that the exclusionary provisions of this paragraph shall apply only to and within governmental agencies, counties, cities and towns which now have or which hereafter may adopt separate laws relating to the licensing, registration and regulating of persons engaged, for business purposes, in any of the areas of trade hereinbefore specified in this paragraph; the exemptions herein being provided to apply only to these items specifically regulated by any such local laws and ordinances; and
Okla. Stat. tit. 59, § 1017