It sounds like a 1985 Professional. Very unlikely that it is 753, more likely it is SL. It is not impossible, though. I have owned two 1985 Merckx Professionals.

There are a few 753 Merckx frames, mostly Gran Prix models. Note the different seat stay (picture from cadre.org):



I have seen pictures of claimed 753 Merckx frames with the script seat stay, but I don't believe decals.