At one time Shimano had a Shimano proprietary brazed on shifter boss. Because of that Campy/Suntour shifters would not fit a Shimano brazeon.
Here's an image of the ^00 Arabesque mount and shifters. It's basically the same as the other Shimano shifters of that period. Note the brazeon mount in the lower left side of the image. The mount base is not square.
Shimano 600 Shifter Boss Circa 1982 by
Miele Man, on Flickr
Cheers