Portugal's Vipers are not the ADF version, although they have the bulge on the tail. They are in fact F-16A/B block 15 OCU, later updated to full MLU standard but with different HF radios warranting the bulge on the tailbase.
The whole concept of the ADF programe was to make the F-16A/B Viper into an Air Defence Fighter for the US ANG, able to carry and launch the AIM-7 with necessary hardware and software updates. The program started ~1987ish and was completed by ~1993. By then a lot of F-16C/D models became available rendering the earlier A/B ADF models surplus, and most were eventually sent to AMARC.
Many of these have since been leased to Italy or sold to Jordan who later gave their ADF's to Pakistan, Thailand etc.
Source: www.F-16.net, best online source for Viper details of all kinds :)