I don't know if you got this done already but I did mine this weekend. The 12v fss had an extra long tab on one side and would not fit in my 5209 pump. I just removed the fss from the covers, swapped the plates that I needed, and put it back together. No biggie. Attached is a diagram of the little booger. In the attached diagram, the tab on the left is stumpy. I removed that plate with the stumpy tab from the the 24v fss and put it on my 12v fss.