Hi again folks:
Here is what I’ve learned (thanks to everyone on the forum):
My MacBook Pro (Model A1278) required (as recommended below) DDR3-1600 MHz PC3-12800.
While Ifixit advises that PC3L-12800 is acceptable RAM for my model (and I know some here have advised that DDR3-L/PC3L have worked in their experience), it didn’t in my experience. So lesson learned and thank you again to everyone who made suggestions/gave guidance.
Note: another bonus? I only had to spend $30 for 8GB x 2, so win/win.