Some very lightweight single pivot brakes, if I'm not mistaken. These look to be BREW brakes, which are customized Dia Compe BRS 200 brakes. According to this thread, the combined weight of both calipers is ~260g with pads. Doubtless one could go lighter with weight weenie pads.

link to thread regarding brakes: BREW brakes - value?

https://www.ebay.com/itm/13471416481...Bk9SR6DGvKjTYg