Vintage Dirt Bike Q & A

Hi,
Maybe you guys can answer a question for me. When I look at the micro for the 1981 YZ250,I see that it took a twin leading shoe backing plate,but when I look at pictures of the bike on the net I see a tremendous amount with a single like the 1980,and on Ebay a ton of single shoe advertised as for the 1981,too compound my frustration I just bought a couple of 81's,and they have single shoes that look like they were always on the bike,I bought both from the same guy,so it seems strange they both would have been swapped!! The frame numbers are correct and it has the large diameter forks??? Thanks for any help