What is this part?

Try Helen Howes’ website as she specialises in vintage machines and accessories and is very knowledgeable.

Let us know when you find out!

I'm intrigued.
Some kind of foot plate for heavy work?

Thank you. It was in this small cabinet.
Here is the reverse.

Could it be a hinge of some kind. Can you post a photo of the reverse please? Might it be part of an accessory box or a table extension?

Buttonhole presser foot?

Presser foot for zig zag?

It might belong to a different machine!

Yes, try a local dealer.

Thank you. Why didn’t I think of the shop.
It’s not in the manual or another book for attachments.
I should have put the measurements.
It’s 3 1/2 inches long and 1 1/2 inches deep.

Nope! Take it to the nearest shop that sells sewing-machines, preferably Bernina and ask them.

Do you have the user's manual? If you don't, you can probably download it.

If this part is not described in it, it may not be a part of the machine. To me it looks rather like part of a broken zip-fastener.