The flat sap was almost universally carried by the guys in my old department when I first walked through the door (1977). Pretty well banned by most departments, nowadays.
I purchased and used (once!) a Bucheimer "Convoy" blackjack; 15-ounce rounded head of solid lead; spring handle covered with braided leather. Swatted a 6'2", 220 pound physical specimen (high on somethin') who was trying to beat down my partner. The dude collapsed like a brain-shot steer! Scared me half to death, 'cause I (and my partner) thought I'd killed him! He survived, but I did learn a lesson: The flat saps were less likely to cause concussions & skull fractures.
Come to think of it, I do believe he learned a lesson, too!