The newer 503 stoves have a handle to open/close the internal damper. On the older [prior to this year] 503's there was an available fixed damper/insert that slid in place in the exit flue area and reduced the opening into the flue..
Just keep trying different postions with the plate,, it will fit in somehow. I have never seen a photo of the fixed reducer installed,, sorry.
Maybe someone with the older model 503 has some instructions and insight.