I believe Maukaman has misread the question, and not seen the bit about the different location of the spring holes. You do indeed understand correctly that there are very few, if any, modern brakes that will work (Paul's cantilevers may, but I'm not in any way certain). Your best hope is to look on eBay for some of the older-model cantilevers that had the separate spring block clamped to the end of the brake boss, like a modern U-brake does (the Paul ones may work the same way). I've only seen brakes like that occasionally on eBay, but then again I haven't really been looking. They don't need any spring holes in the frame. Or you might get lucky and find brakes designed to work with those bosses.