Browsing the source for "cc" in v6 and v7, if invoked with -2 would replace crt0.o with crt2.o. If the -2 were followed by another character (probably intended to be -20), it would use crt20.o and use -l2 instead of -lc. These options seem to be undocumented, and I can't find any source code of these libraries or indication as to what the purpose was.