Correct 3″ diameter x 2mm wide o-ring for cannister of oil filter.
This fits my 1969 S2. I don’t know what other years or models used the same obscure size.
Modern square profile o-rings are 1/16″ square, about 1.5mm, or 3/32″ square, about 2.5mm, or 1/8″ square, 3mm. The largest just does not fit in the groove of the oil filter base. The 3/32″ can be jammed in, but this deforms the O-ring so it leaves a bulge that is difficult to seat. The 1/16″ fits easily, but is too narrow and can roll under the edge of the cannister so it does not seal. These o-rings started out as 3/32″, but I mounted them on a grinding fixture and ground them down to a true 2mm width.