You're right Sheila, the monogrammer was made after the 500 and 503 in 1969. I bet that Singer made the plate for backward compatibility, or had it already for some other attachment. I know one of my "professional buttonholers" came with it.
As to its location preference, I don't know what that's about. The monogrammer went ahead of the 503 into the living room.
I brought the 411 out first because there's no room in the sewing room for me to set it up. The 503 is squished into a corner, and poor light. I finally got tired of running back and forth and brought the 503 out too.
Oh well.
Since they're both out here though, I'm playing with chain stitching on the 503. It's working. I'm just figuring out the logistics of where the guide for the upper thread needs to be to get it consistent.