I think I received this frame with standard Shimano cable stops, but they don't seem to fit. Also, I couldn't found this shape of downtube bosses anywhere.
The problem seems to be with the non-rectangular shape of the "base" part of the bosses. I can't fit the cable stop over the base, there's a huge gap and the stop can rotate around.
EDIT: I've shown the frame to a local framebuilder who's been in the business since early 90's and he said he's never seen this type before.