The serial number, which appears to be 5X Z 6665, would indicate a 1985 bike, and 55 cm seat tube, c-c. The cabling suggests a Corsa Extra.

The interesting part is the seat stays, they are not pantographed. This is unusual, though not unheard of. That would suggest perhaps a Reynolds frame, or custom build. I'm not sure Merckx was using Reynolds in 1985. The full chrome rear triangle is a nice touch, along with the fork. Merckx should be able to give you more info from the serial number.