Custom prototype helmets are helmets that were actually proposed by NFL teams at some point, but never actually used.
Atlanta Falcons – I believe the red helmet was proposed in the 1960’s at the team founding and the white one was proposed in 1974.
Baltimore Bombers – Proposed NFL team from 1993 that never played. The Ravens took their place.
Dallas Texans – The Dallas Texans (later the KC Chiefs) apparently proposed a blue helmet. The story I have read is that when teams were selecting colors, the Houston Oilers selected their colors first. The first Oilers helmet is this exact same Columbia Blue. So, the Texans used red.
Houston Texans – The Texans originally proposed a white helmet. According to ESPN’s Uni Watch, the white helmet even appeared in Madden 2002.
Jacksonville Jaguars – two different helmets (same striping). Proposed at the team’s founding.
Los Angeles Buccaneers – An NFL team that only played one season in 1926. They wore leather helmets. The custom is below is a standard helmet with their logo.
Memphis Hound Dogs – proposed team in the early 1990’s
New England Patriots – The Patriots proposed the following logo in 1979. The Patriots asked the fans to vote for the change. The fans voted no. I am not sure if the stripe color was ever determined.
New York Jets
San Francisco 49ers – The 49ers proposed this logo change in 1991.
Seattle Kings – The Kings were a proposed NFL team in 1972.
St. Louis Stallions – The Stallions were a proposed team in the early 1990’s.
Tennessee Titans/Houston Oilers//Tennessee Oilers
Washington Commanders – proposed in 1965