Drop that sucker in apple cider vinegar for 8 hours then check it and rinse hit with a soft brisseled tooth brush if it need a little more rust off drop it for another 2 to 4 recheck!! Right now it looks like a 40's style lock washer to me, they had little intentions or nipples to catch on that ever they were tightened against!! We can get a better idea once the vinegar knocks the rust off, avoid electrolisis if possible you can def over do it if not careful
Welcome to T Net. Just the location of your find makes me think washer also. Manufacturing ice would have lots of machinery and plumbing, and they might have used rock salt in their production, which would account for the heavy rust. Just a guess on the rock salt, from remembering turning the crank on the ice cream bucket.