In 1973 Frank Williams managed to attract backing from cigarette giant Marlboro and Italian sports car manufacturer Iso Autoveicoli S.p.A. for the 1973 season. The Politoys FX3 was reworked as the Iso–Marlboro FX3B and a second car was built. Two new drivers were signed, New Zealand's Howden Ganley and Italy's Nanni Galli. 

Introduced at the 1973 Spanish Grand Prix, the Iso–Marlboro IR was driven by eight different drivers during the rest of the 1973 season. Ganley was the only regular driver and he scored a point with the car at the Canadian Grand Prix towards the end of the season. Of the other drivers; Galli, Pescarolo, Tom Belsø, Graham McRae, Gijs van Lennep, Tim Schenken and Jacky Ickx, only van Lennep was able to score a point, at his home race in the Netherlands. 

Both Iso Rivolta and Marlboro left before the 1974 season, leaving Williams with financial problems. The two IR chassis were retained, now re-designated the FW after Frank Williams, but initially only a single car was entered for Arturo Merzario, who had replaced Ganley as the team's number one driver.

Activity: 1973 - 1974
Grand Prix: 30
Best Result: 4th - A. Merzario (Italy 1974)
Best Grid Position: 3rd - A. Merzario (South Africa 1974)
Fastest Laps: 0
Championship Points: 6