Originally Posted by KCT1986

Have you checked that the current crank is just a double? The 52/42 rings are standard on a triple for this 105 (FC-55??) and your smaller ring looks that it has shift pins for shifting from a third small chainring. The actual part number should be stamped on the back of the crank arm, right next to where the pedal mounts.

Your RDER is also a longer cage (GS) version of the series normally used for a triple crankset.

Regarding your RDER, it should be rated for a 27 teeth largest sprocket on the cassette. Most people report going a tooth or 2 higher is possible.