My word for it is quick release, or quick set. Others may call it something else. I don't remember what a parts list may call it.

What is it used for?

To release the pressure on the presser foot. Do you have a manual for your Domestic 153? you can find one at http://ismacs.net/domestic/manuals/d...ine-manual.pdf

Check out pages 11 and 51 of the pdf

Another type would be the Revco darn-o-matic like the one in the thread https://www.quiltingboard.com/vintag...w-t273617.html

Janey - Neat people never make the exciting discoveries I do.

I did not realize that the 153 had a similar mechanism! I will have to take a closer look at mine. Do most sewing machines have something like this? I was thrown off because the one on my 31 looks so different.

I would say that most machines have a hollow bolt (thumb adjustqable) instead for adjusting the pressure.. I cropped too tightly on my 153 Domestic to tell for certain. BTW mine is made of magnesium.