Dewalt 20V max batteries VS B&D 20v max

[newbury]

Previously I thought they were incompatible.

But looking at questions over on Amazon I see:

Question: Will this battery work on the 20V MAX dewalt drills since B&D makes both of them?
Answer: No. I have several Dewalt 20V Max products and was hoping to reuse the batteries with B&D yard tools. After researching this, I found a post that showed the 2 batteries side by side and it is apparent that there are physical size and keying differences between the two batteries. The conductors on the B&D are much larger than on the Dewalt batteries. Also the distance between the conductor on one end and the physical release clip on the opposite end are two different distances for the B&D and Dewalt batteries. see less
By PTM on December 8, 2013

Yes, DeWalt made B&D put a little notch on the B&D to make it not fit DeWalt. (They want you to buy the expensive DeWalt version after all) but file off that notch with a dremmel or other tool and it will fit fine. They are identical inside. Source: Personal Experience
By CaptainBasic on January 16, 2015

source here

Has anyone here actually tried the filing down?

The B&D's are half the price.

/edit - I've also found other off-brand batteries on Amazon CLAIMING to be compatible, and I've bought one to try: FlylinkTechｮ Dewalt 20v Max Drill Battery For Dewalt Cordless 20v XR for $40. Reviews indicate they are not quite as good but worth $40.
Next stop ebay, but I've had slight difficulties there.