The original era would likely have been made by See-Rite...I cant quickly find any original clamp-on or repros (White Glove) but if you will be at the National meet in Wisconsin next month, you will likely find them at the swap meet (and you can meet "BigKev" and others, myself included).
CAUTION: These are just generic clamp-ons that were early 40's through 50's but there are several on Ebay just so you can get a feel for what that type looks like and how they attach...I recommend a more diligent search for some See-Rites with a clamp-on: