I'm restoring an original 64 impala SS 409 car right now, and when we were taking is apart my heater box had the same 409 stenciled on it. I have asked several people and no one seems to have an answer. I also would like to know if it has any correlation to it being a 409 car. I don't know how to post photo's on here. but if you send me you email I can forward my photo's of the heater box to you. also since I can't post photo's, go to the chevytalk website and post you photo's and question on there Verne Franz is a very knowledgeable guy he may be able to tell us something. He is also a New Jersey guy.
Bob