Here is a comparison of the two. If you are looking to buy, most sellers don't have a clue what they have, and will say things that are obviously wrong if you know what you are looking for.
Here are both plates. If it does not have that rounded edge, its not going to fit a 66/99 (not if it's Singer, maybe some aftermarket one will, but I've not seen one ).
This is the issue - the hooks on the back side are too far apart on the straight edge style to fit on the needle plate of the 66/99.
This is the machine that plate was made to fit on - a 101. I've got the manual, this part came with the standard set, it's suppose to fit that way.
However because Singer was diligent about putting holes in exact locations - you can fit it on the 201, 15 and 115. For all this style Singers, the distance from the edge of the needle plate to the needle hole and then to the second hole (and even to the bed holes) is exactly the same.
Here is the larger plate on a 15. You can see how the needle plate is quite a bit bigger - but it does fit just fine.
And the proper plate on the 66. It is the same size as the needle plate and covers it completely.
