that is odd. The only place I remember seeing them used is on the Vanguard engines. I pulled that part number up in the briggs dealer site, and the smallest engine it list is a 30xxxx
Thanks for the replies. The engine code is 385777-0349-E1, it"s a 21hp Vanguard twin. There also were washers in the box that were about the same size and made of gasket material.
I have seen both used on various Vanguard engines over the years. Some had the rubber washer, some had the one in your picture. Some used nuts for the valve covers, some use bolts.