Monaco license plate: TT (documented since 1978)

Format, years and the law

Format: TT99, for example TT70 (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: Temporary: short-validity plates (transit, awaiting registration). Issued for car, van, and motorcycle.
Colours: Characters #CC0000 Background #FFFFFF
About this plate: TRANSIT TEMPORAIRE, temporary importation, conditioned by art. 6 on the "convention douanière franco-monégasque du 13 mai 1963" and available for vehicles from outside the Franco-Monegasque customs union. A 99-plate block with a one-year ceiling.
Format notes: Art. 6: "Le numéro d'immatriculation est composé d'un symbole TT suivi d'un groupe de deux chiffres au plus, allant du n° TT01 à TT99." Maximum validity one year.

AM 78-5 art. 6, série TT https://legimonaco.mc/tnc/arrete-ministeriel/1978/01-09-78-5/

The series id is mc-tt. 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-tt")'