If it doesn't have a fatter than usual plug to hold the extra resistors, it definitely doesn't have the mod. But I've had AT853s with fat plugs where the mod was faulty and I had to have them redone, so that's no guarantee.
Did you buy the mics from Sound Professionals originally? If so, email Chris (sales@soundprofessionals.com) and tell him the problem and see if he can help diagnose it.