In the absence of a picture of a size 48 cm, try and measure the effective top tube length, centre to centre, and compare to the Cervelo geometry chart. If the pictured frame is a size 48, I don't think your measurement will leave you with any doubt. If it is a 51 cm and you think it should be a 48 cm, when you measure you will have doubt about whether the tape measure was positioned correctly (if this makes any sense).
I don't recall what a size 48 cm R3 looks like but a size 48 cm S2 has the top tube and bottom tube merging together before entering the head tube.