Looking at my MPF-1B pictures and videos you may have noticed that my MPF-1B didn't have the (optional) Z80 CTC and Z80 PIO installed. They just arrived! Plugged them in and keyed in Experiment 15 from the Experiment Manual: Clock 2 with CTC Interrupt Mode 2, and it worked!
Good to know that I didn't get fake chips this time - I was skeptical, because they look like new. Phew!