You'll find that type on a lot of old French bikes. The hole is sometimes 4.5mm instead of 5mm as I recall, and if so compatible only with some Simplex and Huret downtube shifters' bolts.

There are some products on eBay, clamp-on downtube bosses, that feature the modern standard bosses with the flats and 5mm holes, which is what the whole industry adopted once indexing became universal.