"in rom." If  the  title proper of the work being cited in the 670 is in  a language which uses the roman alphabet, then citations from it elsewhere in  the 670 are assumed also to  be  in the roman alphabet-- so "in rom." is  not needed for them.
But  if they are citations from another alphabet  found  in   the  work,  that  alphabet  needs   to   be identified-- so  "in Cyr.,"
"in Heb.", "in Ara." and  so  on  are used where we normally encounter "in rom."