The convertible version of the sixth-generation BMW 3-Series sedan was spotted recently in prototype form testing on Germany's Nürburgring race track, and we managed to scoop some video footage.

The new drop-top, internally referenced as the F33, is due for a sales launch late in 2013.

What’s not clear is whether the car will be labeled a 3-Series or if it will pick up a new 4-Series badge.

To help further differentiate its two-door models from its four-door lineup, as well as allow for the introduction of a range of new models and bodystyles, BMW is believed to be adopting a new naming nomenclature where its sedan models stick with the odd-numbered names they currently use and sportier coupes and convertibles use even numbers.

This would mean the new 3-Series Convertible may be renamed a 4-Series Convertible, while the smaller 1-Series Convertible could be renamed a 2-Series Convertible for its next-generation. This would also mean that the M3 Sedan would remain an M3, though the coupe and convertible versions would be called the M4 Coupe and M4 Convertible respectively.

From this latest spy video it’s clear BMW’s latest convertible will feature an automated folding hard-top, just like the roof on the model it’s set to replace.

Sadly, however, it appears as though the current model’s strangely proportioned rear is here to stay for at least another generation.

As for the engine lineup, expect the same 328i and 335i designations from the sedan to filter across to the convertible. This means a 240 horsepower 2.0-liter turbo in the 428i and a 300 horsepower 3.0-liter turbo in the 435i model. Transmission choices should include a standard six-speed manual and optional eight-speed automatic.

As mentioned, the new 4-Series Convertible should land in showrooms towards the end of 2013, about the same time the new 4-Series Coupe arrives.