I agree with Michael, it never went beyond the discussion stage. In Autosport for 18th August 1983 the following appeared:
German sources suggest that Stefan Bellof may make his Formula 1 debut in a works McLaren Cosworth at the European Grand Prix next month.
The same sources believe that there is a possibility of Bellof’s joining the factory team as Niki Lauda’s team mate in 1984. The young German does, of course, have a Porsche contract – and Porsche build the TAG turbo engines ...
The following week’s issue contained this item:
Stefan Bellof had a good chuckle over the rumours in last week’s magazine that he might drive a third McLaren at the Brands European Grand Prix.
“I think it would be difficult as I am a Rothmans sponsored driver, and they are backed by Marlboro,” said Bellof, who nevertheless didn’t deny that McLaren was one of the many teams that he had spoken to during the course of the German Grand Prix at Hockenheim.
“Sure, I spoke with a lot of teams there, but I honestly think it will be very difficult even for Jonathan Palmer to get a drive next year, let alone me.”
There’s no further suggestion in Autosport of Bellof driving a McLaren in the race, and he wasn’t listed on the entry list. There were, though, several further rumours linking him to the second McLaren seat for 1984, and he did, as you say, test for McLaren in 1983 in the famous Brundle – Senna – Bellof test session
Edited by Tim Murray, 23 August 2015 - 09:20.