Something about the case speaks more to me. With the angles and lugs, smaller bezel, in yellow gold has a vintage feel. Also think the enamel work is different, more detailed for sure. Have heard the enamel artist shifted from mother (for 5131j) to her son for later models including the 5131p and 5231j. So styles are different.
I just got my 5231j and love it. A true milestone! Glad I’m part of the WT enamel family!