You could always just use their correct names. The Plomb Tool Company, of Los Angeles, makers of mechanics tools mainly for the automotive and aviation industry, was spelled with an "o". Fayette R. Plumb, Inc., of Philadelphia, makers of striking tools, especially, hammers, hatchets, axes, and picks, but also blacksmithing tools, stoneworking tools, and some knives, is spelled with a "u." There's no real need to use the plumb-bob looking symbol or even a representative "v" (i.e., PLVMB), except maybe when discussing the difference between that branding in the mid 30's on and the early years, when the "o" was indeed just round (i.e., PLOMB). But rest assured, the easiest way is just Plomb and Plumb.