Thanks Andy, saved me digging into my files!
The reason for the canvas trunking is quite simple - it's a mock up, and is part canvas, part linoleum, which has suffered and degraded since the splendid work was undertaken by the chaps at RAF Shawbury in the 1960's.
This, the only (almost) complete Typhoon in the world, was sent to the USA for evaluation during WW2. After brief trials, it was put into storage, and, after the war, it was earmarked for what eventually became what is now the aviation collection of the Smithsonian, but was again put into storage and never actually got on display.
It was returned to the UK in the early/mid 1960's, in exchange for a Hurricane, when it was intended to be a future exhibit at the then planned RAF Museum, which din't open until 1972.
During it's time in the 'States, many parts and fittings 'went missing', including all the engine cowlings, the spinner, radiator and associated trunking, an aileron, I think at least one flap, some other engine fittings, and a host of smaller parts.
A team at RAF Shawbury, Shropshire, UK, made plain, mock-up cowling panels, which lack the internal frames and are in one piece, instead of the multi-panels, and also lack external rivets and 'surface detail', and they also fabricated and fitted the basic shape of the missing control surfaces.
The missing engine and associated parts were, of course, hidden under the 'new' cowlings, but a radiator front, at least, had to be visible, and this was 'mocked up' by cutting down a truck radiator, to 'look the part'!
A spinner couldn't be found anywhere at the time, as all Typhoons were scrapped immediately after the war ended, so the team did a really splendid job of getting as close as they could, by cutting, trimming and joining a spinner from a Hastings - hence the slightly pointed, 'wrong looking' spinner in place to this day.
As for the missing radiator trunking, visible at the rear of the 'chin', at the open rear end, this was fabricated from canvas, cardboard and 'linoleum', and painted to look like the metal trunking - good enough to fill the otherwise empty space.
The cockpit has some 'non-standard' items, such as the American - fitted, yellow tape around the Blind Flying Panel instruments, and an American documents case, not seen on 'in service' RAF Typhoons.
I was hoping to ask the team at The RAF Museum Conservation Centre, Cosford, where the Typhoon was taken to in October, for display in the Cosford museum site, if a 'proper' restoration would take place before the aircraft went on display, but ran out of time during my visit. However, with the recent news that the aircraft is now being loaned to Canada this year, for a period of possibly up to three years, any such major fabrication work, at this stage, is highly unlikely.
Many modellers, and indeed kit manufacturers, have been 'caught out' by referring only to what is seen on this particular aircraft, and duplicating it, when, with a little simple research, especially by manufacturers, any errors could be avoided, as the information regarding this airframe has been published, in aviation and modelling magazines, a number of times over the years and is, of course, available from the RAF Museum.
For the individual modeller, there is an excuse, but for authors and publishers of such things as 'Walk Around' guides/books, there is no excuse, as it only takes a little effort and few questions to establish the facts, and present the information that people are paying for, when buying such 'guides'.
Hope this helps.