This is a USAF spec internal wrenching bolt:
https://www.skygeek.com/military-sta...rilled-hd.html

Here's a DIN912 cap screw:
https://4.imimg.com/data4/SG/TV/MY-2...ws-500x500.jpg

or smooth side version:
https://img.accu.co.uk/products/SSCF-A2_lg.jpg

Tom has a point in that most linear ("V") brake anchors do
not have cylindrical heads and actually do look more like an
'internal wrenching bolt':

https://i5.walmartimages.com/asr/9e1...0&odnBg=FFFFFF

p.s.
A metric DIN #912 cap screw is usually under a dollar. Not
so with Actual Bicycle Parts!

https://www.walmart.com/ip/Tektro-An... 34&tmode=0000


--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971