"according to eBay" every Singer part is for a Featherweight! I've seen lots of non-singer parts listed as "may fit a FW" too. Sellers on ebay have the idea that if they mention "featherweight" in the listing, they can double the price.
Actually, very few parts were specifically for the FW - and there are very few side clamp Singer attachments that are machine specific -and most that are - is because of a feed dog plate or cover.
I once saw a basic Singer buttonholer that someone put in a black fabric case that the White units come in. Seller listed it as "Rare Featherweight Buttonholer" and wanted $100 for it. I guess since it came in a black case, it belonged to a machine that came in a black case. Much cheaper to buy a regular Singer unit and a regular White unit and swap cases.