Air dome
That part is called an air dome. It is present in every belt drive washer, full size and portable, from at least the early 1970s, maybe earlier, to the end of the belt drives in April 1987.
Is this part cracked or damaged? If not, it's probably the seal that goes with this that is leaking not the dome itself.
Wash the dome completely, and remove ALL the silicone from the the seal and the tub wall. Clean any deposits from the seal and the tub, there is a good chance a new, solid connection to the side of the tub without residue or debris will make a good leak free seal.
If not, the part number is 360804. There should be new ones available.
Gordon