Here’s what WikiPedia says about this: “Local license plates were issued at least since 1901, which usually consisted of the full name or the abbreviation of the name of the district, followed by a serial number. (e.g. “LISBOA 123” or “LXA. 123” for a vehicle in Lisbon).
A national car registration system was established in 1911. The country was divided in five zones (North, Center, South, Azores and Madeira), each having an identification number sequence for the license plates, respectively N-000, C-000, S-000, A-000 and M-000. In the mid-1930s there were already so many vehicles registered, especially in the South Zone which included the City of Lisbon, that the identification number was already up to five digits. As this created some difficulties in the identification of vehicles by authorities it was decided to change the system. Should you want to get more data, then please do not hesitate to use universal license plate number lookup tool.
In 1937, the second Portuguese car registration system was established. This consisted of the sequence AA-10-00 to ZZ-99-99. The division in zones was kept. The letters AA to LZ were reserved for the South (Lisbon), the MA to TZ to the North (Oporto) and the UA to ZZ to the Center (Coimbra).
The vehicles registered with the old system had to change to the new one. In Lisbon the vehicles of the series S-000 and S-1000 changed to the series AA-00-00, those of the S-10000 to the AB-00-00, the S-20000 to the AC-00-00 and the S-30000 to the AD-00-00. In Oporto the changes were from the N-000 and N-1000 to the MM-00-00 and from the N-10000 to the MN-00-00. In Coimbra all vehicles changed to the series UU-00-00. In the new licenses, the vehicles kept the previous numbers, with the exception of those of the tens of thousand (e.g. the vehicle C-123 would change to UU-01-23, the N-1234 to MM-12-34 and the S-12345 to AB-23-45).
The letters MG, and later ME and MX were reserved for the Army, AP for the Navy, AM for the Air Force and EP for the government civil ministries. The letters CD, CC and FM were reserved for diplomatic vehicles (respectively for diplomatic, consular and non-diplomatic personnel), but with red characters on white ground. The letters TA, TB and TC, in red plates with white characters were reserved for temporary licenses respectively in Lisbon, Oporto and Coimbra. National Republican Guard and the former Fiscal Guard had special sequences beginning respectively with the letters GNR and GF.
In the Azores and Madeira the new system was only adopted in 1962. By this time the Azores were themselves divided in three registering districts: Ponta Delgada, Angra do Heroismo and Horta. In this case the groups of letters AN were reserved for Angra, AR and AS for Ponta Delgada and HO for Horta. For Madeira were reserved the groups MA and MD. The white letters on red TD, TG, TH and TF were for temporary licenses respectively in Ponta Delgada, Angra, Horta and Madeira.
In the beginning of the 1980s it was decided to end with reservation of letter sequences by zones, as a result Lisbon registered vehicles used sequences that were reserved for Oporto from 1982, with the NA to NL sequence, or from 1987 the sequences OA to OL, PA to PL, RA to RL or SA to SL, and from 1990 of former Coimbra reserved sequences, for instance, UA to UL or XA to XZ (the latter being the last left side letters sequence used). Oporto letter sequences, in fact, always consisted of a combination of the two letters, both alphabetically equal or above M.” I think it is enough. Next time I will extend my thought about this topic. Anyway, I’d like to hear your comments and suggestions. Thanks a lot.