Like TenGrainBread, I suspect these were converted from standard centre-pull models. Brazed-on (aka direct mount) centre-pull brakes, while relatively rare, eliminate the weight and flex of the brake bridge. The latter increases both braking power and modulation. The downside is the added cost of the brazing operation for studs. One can argue the quantitative benefits but it is the nth degree type of detail that appealed to constructeurs and their high end clients.
Typically, when a company manufacturers a direct mount component, they also supply the brazed-on fitting to guarantee compatibility. This is why companies like Campagnolo and Shimano offered dropouts and shift lever bosses. Consequently, if a company offered direct mount brakes, you would expect to see matching bosses and this is the case when you look at the cantilevers in a MAFAC catalogue. Dia-Compe catalogues show brazed-on fittings for both their cantilever and centre-pull models (see scan) but the entries for the centre-pull brakes makes no specific mention of direct mount versions. This suggests that Dia-Compe considered demand for direct mount centre-pull brakes to be so small that they were either non-catalogued special orders or manufacturer conversions. Often, with niche products, it's less expensive for large volume manufacturers to offer a standard product than change the manufacturing, packaging and logistics process.
I have seen relatively few Universal catalogues but none that I have seen include direct mount fittings or brakes, either cantilever or centre-pull.
As an aside, I distinctively remember seeing my first direct mount centre-pull brakes, as it made such as impression on me. They were Dia-Compe on a 1976 Centurion Pro Tour.
It's also worth noting the current trend towards direct mount dual pivot brakes as an option to disc brakes, though the forks and stays use recessed threaded fittings, as opposed to protruding studs, as recessed fittings are more appropriate for carbon fibre frames.
