Since shirring plates are often under the ruffler in pictures and hard to see- here are some of them. And most directions don't even show a picture of the plate - they show it covered by the fabric being shirred.
At the top is the ruffler with the shirr plate detached. You always use a shirr plate with the ruffler, either the one attached if your ruffle edge is narrow, or one that fits underneath the fabric if your ruffle edge is wider. If you are looking at really old rufflers, make sure the shirr plate is there. I've got several with it missing (and several ones that came with no ruffler).
The first two are for a 15/201 and a 66. The 66 is a bit smaller. These two are not puzzle box shirring plates -they are later styles that you'll occasionally see.
The next one is a shirring plate for the 15 style puzzle box. There may be more - but I only have this style 15 box.
The second two are for the 27. The second of these two is an older vintage and goes with a longer style ruffler. the first of these two is the most common, and I think it was used post puzzle box era too.
Last is another older style. This one is NOT for a singer, but it is the style I wanted to show. The Singer one is the same size as the front plate for the 27, and has the shirring part attached to it similar to how this plate is (it's for my White made New Queen).
To give credit where it is due - I've spent a lot of time looking at the pictures on Needlebar to figure out the shirring plates
http://needlebar.org/main/puzzles/index.html
And I belong to the Singer Attachment group on Yahoo - they have a lot of pictures also.