The Plomb Tool Company was still the owner. They did not change the name of the company in 1949 and were never compelled to do so by the Plumb lawsuit. (The company was still known as the Plomb Tool Company until 1957, when they changed the name to Pendleton Tool Industries, Inc. And the company was never known as the Proto Tool Company. The name "Proto", which PTTI continued to use as a brand, became the name of an Ingersoll-Rand Division when Ingersoll-Rand bought PTII in 1975.) They had to stop using "Plomb" on the tools or anywhere else as a brand, rather than as the full name of the company. If the Plomb Tool Company was sending out new boards with only the newly TM'ed "Proto" sticker on it - which your board appears to be an example of, it would've been 1950 through 1956.