There's no mention of "Silver Streak" anywhere in the 1948 sales brochure.
Harry is correct in this! "Silver Streak" was a slang term invented decades ago to describe Pontiac's with those chrome "silver streaks" down the hood and trunk.
Incidently, that silver streak trim motif was developed by an entrant in the Fisher Body Craftsman's Guild contest back then. His name? Donald Burnham, who later grew up to become the CEO of Westinghouse in the 1950's (GM retained the right to use any styling motif presented on a FBCG model entered). Don Burnham grew up in the house next door to where I grew up in West Lafayette IN--his Dad was the longest serving mayor of WL, something like almost 40 years.