Speedmaster Mark iii Authenticity Check - Please Help

Dear forum experts,
I have a question regarding the authenticity of my new purchase Speedmaster Mark iii.
As a new collector I always buy from dealers and distinguished auction house for better QC.
Recently I bought this from an auction house, everything looks ok to me. After I opened the case back, I realized that there is no serial number on the movement.
I crossed compared other cal 1040 pics online, the serial numbers are usually engraved on the rotor but my piece shows "R26972" instead of the normal 8 digit number.
Attached are the pictures of the watch, please advice if it is real or a fake, so i could report and return it.

Photos here.

That is legit. There are different schools of thought about the R numbered movements but it is generally believed that they are replacement movements or rotors that Omega put when the original movement or rotor was too far gone. Those movements are 100% original Omega movements. Nowadays they would just restamp the movement with the original serial number but back then since they stamped the SN on the rotor it was more expedient for them to have those rotors ready to go with a replacement number.