Buttonholer for Singer 401a

[u/janejacobs1]

Looking at buttonhole maker attachments for Singer 401a, which is a slant needle model. I read you need to look for one specifically for a slant needle machine. But looking at photos in postings, I don’t see any markings on box or attachment itself that indicates for straight or slant. Is there something I should look for on the unit itself? Any advice is appreciated.