Monaco license plate: Trailer (documented since 1978)

Format, years and the law

Format: A999, for example A812 (9 any digit)
The format above follows the issuing rules: a registration that fits it is one this series could have issued.
Years issued: Documented since 1978; the format is likely older than the first year documented. Still issued.
Vehicle class: Trailer: trailers and semi-trailers where separately serialized. Issued for trailer and semitrailer.
Colours: Characters #5B9BD5 Background #FFFFFF
About this plate: TRAILERS over 750 kg. The A block is exactly 999 registrations wide, which is the smallest closed number space in the ordinary Monegasque system.
Format notes: Art. 6: "une lettre et un groupe de trois chiffres au plus, soit du n° A001 au n° A999." The letter is fixed at A, which is why A is absent from the car letter-prefix alphabet. Only trailers over 750 kg carry their own plate; lighter ones repeat the towing vehicle's (Code de la route arts. 92-93).

AM 78-5 art. 6, remorques https://legimonaco.mc/tnc/arrete-ministeriel/1978/01-09-78-5/
Code de la route arts. 92-93 https://legimonaco.mc/code/code-route/

The series id is mc-trailer. The API returns its format, validation, colours and designs with the rest of Monaco's series:

curl -s https://vehiclesdb.com/api/v1/plates/mc \
  -H "Authorization: Bearer $VEHICLESDB_API_KEY" \
  | jq '.series[] | select(.id == "mc-trailer")'