Hoi there mates, it seems to me... that not all of the RBAF vipers have the IFF. Been looking around and it's hard to tell... as far as I know #111 (first viper delivered) and #101 do not have the IFF antennaes. The rest does.
The Peace Crown II jets were delivered with the AIFF system and antennas (Bird Splitters) the PC I jets were later modified with the system back in 2001, how do I know? I was there training the PC II Crew Chiefs while the MOD was being done.