Do I Need to Return to Immigration in December to Show My Financial Situation for My Non-O Visa?

Chaengwattana does NOT hand out that paper, that's why you don't have it ;)

Only a FEW immigration offices have the "return in 3 months to show your updated bank book" policy.

The way you tell if the immigration office you use has that policy or not IS

Did they give you a notice that had a return date of 3 months on it saying to update your bank book and come back?

IF yes, you DO update your bank balance and go back

but

if they didn't give you that paper you don't.

They will check your bank balance NEXT year when you get your extension to make sure you followed the terms of the extension (800K baht banked for 3 months after you get your extension and then the balance can't go below 400K the rest of the year).