According the the 1940 Census, Alphonse M Plomb was 69 years old and a toolmaker living in Pomona, CA. He was born in France in 1871. In the week of March 24, 1940 he had only worked 16 hours so I suspect he was partially retired.
https://familysearch.org/pal:/MM9.1.1/K9W2-XV8
(You can click and see the actual Census record from this link.)
Per other site on the web, parted ways with Plomb tools around 1917 but continued to make some tools under the name A. Plomb.
Alphonse Plomb is a French name so the French pronunciation would most accurately represent how it was pronounced within the company and would be the correct pronunciation. However, it is likely that the name was widely mispronounced as it was foreign and recorded communications would not have been available for something trivial like a tool brand. Also, considering Mr. Plomb had already left the company when it really started to grow, everyone else there could have anglicized the name.
Interestingly enough Plomb in the french word for the metal lead but can also be used for shot and sinkers like in the USA.
There are a number of places on the web that will allow you to here the French precipitation and I listed a few below. I would best describe the pronunciation at "plohm" with much less emphasis on the 'p' than we would use in the USA.
http://translate.google.com/transla...a=X&ei=8SuUULb8LeTU2AX7qYDoAg&ved=0CB8QrgYwAA
http://www.forvo.com/word/plomb/