The Plymouth Belvedere Convertible holds the honour of being the longest lasting Budgie Miniature which had no changes throughout its life. It first appeared in 1957 as Esso Series No.20 Plymouth Belvedere, the last model in the Petrol Pump range, and then continued in production under Budgie until 1962.

The real car which inspired the model was the 1957 Plymouth Belvedere so when No.20a came out it was very up-to-date. Constructed on a body & base design rivetted together, the body had a full frame for the windscreen whilst the driver, steering wheel and the seats were part of the base section. The chunky-style wheels, used only for this model, were unpainted metal.

The body was a pale pink colour which is known to vary a little from pink to creamy beige but this may be due to fading over the years to which this model is prone. A white version is claimed to exist but this is not certain.

Silver trim was added to the grille, headlights, front and rear bumpers and sometimes the rear logo in the centre of the boot (trunk). The base was red and had 'No.20 Plymouth Belvedere' and 'Made in England' underneath. 

After the Esso Petrol Pump Series ended, No.20a appeared in 'Mobile' and 'Modern' Vehicle Series boxes and was included in Budgie Gift Set No.12. 

Never a particularly successful model, the Plymouth Belvedere Convertible was withdrawn in 1962. It was replaced (in 1968) by No.20b Emergency Vehicle.