Lo and behold! It fits my Canon (5D Mark III) and also triggers from!
Another advantage: I have ordered this on my suspicions because I was looking for a way my two Speedlite YN-560III trigger while on the camera YN-E3-RT sit on it. Have among the YN560-XT mounted a servo flash release and can now visually trigger both manual Speedlites by a YN600EX-RT.
Can only recommend this product!