1) Is the 24mm internal thread on Ak74 brakes common for "authentic" combloc brakes ?

24mmx1.5mm rh is common to russian and bulgarian ak-74's and ak-10x series. 22mmx1.5mm rh is common to romanian ak-74's.

2) If so, then why go with instead of 14mm ?

to make it possible to easily clean the large primary exspansion chamber of the ak-74 style brakes.

3) Furthermore, is the 14mm barrel threaded and a adapter screwed on or is the FSB have the 24mm threads on it ?

the fsb is an investment casting and has the 24mm (22mm in the case of romania) threads machined onto it. the barrels are unthreaded.

the 14mmx1mm lh threads were used on the earlier ak-47 and akm models...and that thread is universal to all conutries that produced tose models (exceptions being rare, israel/galil and i can't recall if the s. africa/rand r4 offered two threadings or just the metric).