There has been a lot of interest in this subject over several months.
The Bernina piping foot Philippa uses has no number and was made by Bernina UK in their Cardiff workshop. They bought blanks from Bernina Switzerland to do this. They are no longer doing this and have not for some time.
Some people have old ones stashed away that they might have forgotten about and other machine brands do make them (Pfaff make a good piping foot).
If you are a Bernina user like Philippa, she suggests your best bet is your zipper foot. In Philippa's opinion the 12 bulky overlock foot and piping foot 38 are no better than the 4 zipper foot - and you will be sure to already have this.