The story is a bit vague, but the question was asked again at this year's BotCon where the special guest was Hideaki Yoke.
From OTCA.com.au round-up:
'He also talked about the Origin of the Optimus Prime/Convoy toy. It was intended as the leader of the Diaclone series, so had to be something special, but the first designer appointed to the task, didn't finish it due to his obsession with Sports Cars. A team was then put together to finish the design, which means the classic, iconic toy can't be creditted to just one person.'
He also commented about it at the 2005 BotCon.
From transformers.wikia.com:
'At BotCon 2005, Hideaki Yoke revealed that Optimus Prime (or more accurately, Diaclone Battle Convoy) was not designed by a single person, but rather a team of Takara designers as the one who was originally assigned the job had fallen behind. It is said that a young Shoji Kawamori was also a member of this team at some point, who would design THS-02 Convoy over 20 years later. The patent for the toy, however, lists Hiroyuki Obara as the designer.'