So the solution to using an AF-B on a 5D3 is to buy an Ed-C from Canon (less than $20 delivered), take the mounting piece off (four size 00 philips screws), remove the mounting piece from the AF-B, and put the piece from the Ed-C on. The screw holes in the opposite corners will line up perfectly (lower left and upper right when looking at it).
thank you for your clarification.
After thinking a while and using Google a lot I came to the same conclusion: To mount the Ed-C onto the AngleFinder-B and maybe squeezing the screw holes a little bit.
Even after searching a lot on the Canon website or even the worldwide google search, I haven't found a single source where to buy the adaptor Ed-C. The only result I can find is a focusing screen called Ed-c, but no adaptor to the AF-C.
It might be that I am "partially blind" and just could not find, where others just make three klicks and say "here it is", so I might need a hint where to look Do you have a good source / link to a website at hands?
You're not blind - Canon makes it exceedingly difficult. I traded email with customer service for a week before I got the right answer. This is what they sent:
"The Canon Direct Store does not sell parts. If you wish to purchase parts, please contact our Jamesburg Parts Order Center. They can provide you with the price and availability of the part or Ed-C Adapter you're requesting. You may call them at (866) 481-2569 between 9 am and 7 pm ET, Monday through Friday (excluding holidays)."
So if you are in the US, call that number. The part number is YG2-0430-000 ADAPTOR ED-C (MECH) and it is $12.16 plus $6 postage (and sales tax).