Just noticed that, at B&H Photo, the AB mount Flyer is $600 less than either of the other two mounts. Any idea why?

Nope, they're the same price.

Stock Flyer with AB mount: http://www.bhphotovideo.com/c/product/328176-REG/Steadicam_F12ABNN_Steadicam_Flyer_Stabilizing_System.html

Stock Flyer with V-Lock mount: http://www.bhphotovideo.com/c/product/328174-REG/Steadicam_F12VLNN_Steadicam_Flyer_Stabilizing_System.html

Stock Flyer with NP-1 mount: http://www.bhphotovideo.com/c/product/328175-REG/Steadicam_F12NPNN_Steadicam_Flyer_Stabilizing_System.html

Maybe you were looking at a V-Lock mount "kit" which comes with batteries and such, and then just the stock Flyer which comes with no batteries for the AB one.

EDIT: I just did the "email me a better price" thing. That's a huge discrepancy. I'd call B&H to confirm that that's not some type of error.

Well, they've been sending out the AB price for at least a week. Quite a deal, huh? Even if AB stuff is a bit more expensive.