Same one I used. You don't need to get these from eBay, if that's where you're looking, Cousins in UK and Ofrei in USA still sell them. The one you're looking at is nearly 7 years old if I read the code correctly not that it should be a problem.
These are actually a slightly convex shape and the SM300 dial is flat so, it is not perfect but you don't notice it. I think the correct one ends in 'A' and is flat but, I don't believe you can get these any more.
I think the 1500N is not the correct one as it has a curved surface. The 1580N is the flat one and is still available, or at least Al has just ordered one for my 166.068 and a couple for himself for SM300's.