The 1990 catalog shows the ST1000, ST600 and ST400 all have forks with low rider bosses:
https://vintagecannondale.com/year/1990/1990.pdf
Only the ST1000 included both racks. Photo of the ST1000 shows the low rider front rack mounted, with the top of the rack mounted at the boss.
I'm wondering if there was some alternate system for mounting the rack, where the boss didn't need to be threaded.