The interesting thing about sideplates is the variation. It looks like they were incorporating changes in the production as they went. 

I went many years without seeing a sideplate, then bought 4 & passed on 3 others withing a 2 yr span. Now I've gone 5 yrs without seeing any others! Kind of like Remington #7's, saw maybe 6 of them within a yr & none since. Maybe the lesson is - get'm while you can.
Regards, Ron