I'm going to agree that the "bomb" suggestion for the pronunciation is correct.
The reason for that is because Proto's own official history, written for their 100th anniversary in 2007, says Alphonse Plomb and his two founding partners--Jacob Weninger and Charles Williams--were all German immigrant blacksmiths.
I'm not going to try to figure out how Charles Williams' name could be German,

but Alphonse is a given name of primarily French or Swiss origin, so Alphonse Plomb may have been a German immigrant of Alsatian, Bavarian or Tyrolean descent. In any case, he was from somewhere in Germany, as was Henry Lomb, one of the German immigrant founders of Bausch & Lomb.
You can hear a pronunciation of "Bausch & Lomb" here:
http://thenameengine.com/name.aspx?id=2131&name=Bausch-and-Lomb&cat=208#.Ufs33k3n9Ox
Just add a P and you get "Plomb" to rhyme with "Lomb" and "bomb."
On the other hand, I've heard an awful lot of people pronounce "Bausch & Lomb" as "Bausch & Loam," so who knows...