What is the proper name for this type of washer?
Using google image search, your photo is used on a UK company's web site:

https://titanclassics.com/product/m6-washer-spacer

They call it a washer spacer, however, that term is not very definitive:

https://duckduckgo.com/?q=%22washer+spacer%22&amp;iar=images&amp;iax=images&amp;ia=images

https://duckduckgo.com/?q=%22spacer+washer%22&amp;iar=images&amp;iax=images&amp;ia=images

https://duckduckgo.com/?q=washer+spacer&amp;iax=images&amp;ia=images
More definitively, they are called "flange bushings" or "flanged bushings":

https://duckduckgo.com/?q=flange+bushing&amp;iax=images&amp;ia=images

https://duckduckgo.com/?q=flanged+bushing&amp;iax=images&amp;ia=images

Somewhat less definitively are "step washers" or "stepped washers" and  "shoulder washer"

https://duckduckgo.com/?q=step+washers&amp;iar=images&amp;iax=images&amp;ia=images

https://duckduckgo.com/?q=stepped+washers&amp;iar=images&amp;iax=images&amp;ia=images

https://duckduckgo.com/?q=shoulder+washer&amp;iax=images&amp;ia=images