That's confusing indeed, as it's hard to imagine that a car of a famous make would not be mentioned in regard to a small series where mostly little known makes competed. Especially when you say that Van Diemen is a lesser known company compared to this one. Indeed, it looks even more to me now that this car might have been a conversion from a car for another series. Then again, I'm totally guessing now again.
Browsing through Formula Monza/Panda/F.I.R.E. information I've stumbled upon Marcos name. Any chance it is Marcos (if it was THAT Marcos)?