Engine & frame numbers on these models were all the same & were preceded by the model code. The N suffix was used in 1950 & NA for 1951/2. 350/500 twins all used an H prefix. The 650 models with the duplex frame used a D prefix & unit 650 duplex used a DU prefix. Additional suffixes were used to designate model type or condition eg. TT (model type), C (high compression) or P (police). Some of these only appeared in certain markets.

During 1969 a new number system was introduced which used a two-letter prefix indicating month & year, followed by the model type & the serial number. The first letter is the month, the second the year. This system used the Triumph model year not calendar year, which ran from August of the previous year to July of the current year. From July 1980 a third letter A was added to allow the system to keep going. Many model types included a V which designates a 5 speed gearbox.

For example: T140V HK12345 - is a 5 speed T140 model built in July of model year 1974/75.

Also; T140E NDA12345 - Is a T140E model built in October of model year 1982.

In addition; suffix D indicates de-luxe twin, suffix SS indicates Sports Special twin, suffix Q indicates quietening ramp cams (some 1947 singles)