I don't know, to be honest! That's a good point. I think it'd be best categorized as a 'wink'. I doubt Tony purposely created his mini-reactor to look like the windows in a home he may or may not have lived in at some point in his life. On the other hand, Howard Stark is totally the kind of person who, when building the big arc reactor Tony based his mini-one on, would decide to make it like the decor in that one house he owns, because it needs to be pretty as well as functional. Or, Howard designed the windows too, and just has a thing for circles. In which case, I wonder what Mrs Stark's engagement ring looked like... :p