First....READ THIS...
https://www.pegasusautoracing.com/docum ... =TECH00136
That all being explained....the spare Beck Arnley 072-2538, 13/16" rear brake cylinders (for the Datsun 510 ) that I have, but never actually installed have a threaded hole (female) with a CONVEX "nipple" down inside of it, that would mate up to either the flared, CONCAVE end of a brake line OR adapter fitting. The end of the adapter fitting that screws into the wheel cylinder will be male, with a CONCAVE face. The other end of the same adapter fitting will be female, with a CONVEX "nipple" down inside of it (just like found in the brake cylinder itself), to mate up to the CONCAVE end of a brake line, and receive the male, brake line nut...I can take pics to show, if needed, but hopefully my explanation will help...
Part numbers for the brass adapter fitting vary....I bought mine at Advance Auto Parts # 327851, other parts numbers from NAPA, etc. could be different, which is why I thought a detailed, physical description might be useful...