Here's some copied-and-Google-translated text about the car from a Spanish site:
The Milton (also known as "Milton Seat" or "Seat 600 Milton"), is one of the derivatives of Seat 600 that emerged in the decades of the 60s and 70s in Spain. The Milton is considered the first domestically manufactured spider produced in series (although it was very limited), as previously only had been achievements Serra bodybuilder that stayed virtually as prototype. Milton was presented at the Barcelona Motor Sector which was held in March 1967.
The body design of fiberglass of Milton was the work of Antonio and Pedro Mata brothers, Vilafranca del Penedes (Barcelona). To incorporate the mechanics of Seat 600 D in a convertible car, the chassis had to be reinforced and suspension and brakes also had to be modified. 4 shock absorbers were replaced with higher strength, and rear springs and the front spring were also reinforced. Brakes, although they maintained their original configuration, jaws were optimized with larger surface.
In Milton coexisted the other elements of both models of Seat as other brands such as Renault (R8 taillights) or DKW (headlights).
The price of Milton was very tight, costing 105,000 pesetas the normal version and the version with 119,000 pesetas prepared engine. It was almost the same price of Seat 850 Coupe in its standard version.
The production was very limited and between 1967 and 1968 on the autobastidor Seat type BU, a total of 36 units were built apart from the initial prototype.
