Joe,
If the Lightweight is a mystery to you over there, then how many of them ended up in the UK?I have had no joy whatsoever in trying to locate blades over here and to be honest I was more than surprised when I saw that they were made in 1977 as apart from the blades the one I have is in immaculate condition (it even still has all the original stickers on it!)
My father (who gave me the mower in the first place)has a very early Victa mower stored in his garage that has been there for years if you are interested i could dig it out and send you any numbers I can find on it and maybe you could identify that one for me?
Peter