This standard establishes the rules for the simplified representation of all types of center holes and for their designation on technical drawings. This simplified representation can be used in technical drawings in all cases where it is not necessary to show the shape and dimensions of the center holes or where it is sufficient to refer to unified center holes1'.
UNI 8189-1980 history
1980UNI 8189-1980 Technical drawings. Simplified marking of center locating holes