Red Sea Sharks: Rally organized by Vagabond Car Club?

Although… the cars are all numbered, like a race…

Just a little thought on this, I suspect that the consensus that the event is either just a display or simply a get together is quite likely, but I suspect that they might be participating in an motoring event. The cars have a huge variety in power, so we can rule out the fact that they're going to curcuit race, besides the grounds (and the particpants) would be ruined. Its also very clear that this is a privateers event, not an official sporting event - e.g no roll bars, crash helmets or judging by the image any real organisation! It's more likely that they're going to participate in some sort of time trial around the grounds - this is a test on the drivers agility and ability to maneuver the car around a pre-determined route - the vast walk way to the front door is clearly large enough to accomodate a run, so in this instance a smaller car with useable but not outright power should win - the same logic can be used for hill climbs - weight ratio and driver skill is key.
I had great fun naming the cars when I first read this when I was younger - it was the first tintin I read (at the grand age of 6) and firmly remains my favourite (but I was somewhat disappointed that Skut wasn't a regular charaacter though) I also loved the chaos and detailing in the image - a real treat to be rewarded with such a finale. The cars are as follows:

1 MGA
8 Porsche 356
Red Messerschmitt
16 BMW 501
2 Alfa Romeo Giuletta
17 Cadillac Eldorado
11 Mercedes 190SL
3 Peugeot 403
9 Auto Union 1000
5? Citroen 2CV
7 Might be a Plymouth Fury
36 Triumph TR3
24 Volvo PV544
4 Isetta
Citroen DS in yellow
10 Opel

The grey and red cars at the bottom right are a bit more of a struggle to guess - perhaps the grey one is German marque, with the red one being a Fiat of sorts? I can't make out the nose of the yellow car on the top left hand corner, but the centre left car in red is a Jaguar MK2.

Just out of personal interest and aligned to my ability to draw cars, I updated the image to include equivalent cars from the 70's and 80's for an art project a few years back.