Serial number question
 

16 gauge SxS, hammerless, ejectors, double triggers, serial number on upper is 73*@#, serial number on lower is 180*@# (last 3 digits exactly the same for both). It's the model with the engraved dog and birds and scrollwork. I'm new to this, but logic states that if the numbers are the same, it all belongs together, and if the numbers are different then it's parts from 2 different guns. It's been in the family for 90+ years (serial # on upper is a 1918 build), gifted to my great-grandfather. Any knowledge thrown my way would be greatly appreciated.

As you stated...
If the numbers are different, then the parts are from different guns.

If you could tell us the grade, and barrel steel (marked on top rib inscription) it would be easy to determine if your gun is a 73,xxx vintage, or a 180,xxx vintage.