Your post is a very good example of why I try to be scrupulous in restricting my assertions to relate ONLY to the Works 240Z & 260Z rally cars. I've been studying them for many years and I think I have a pretty good handle on them by now. I know a few examples of what we might call 'skullduggery' in period, but any number-swapping wasn't actually Nissan's direct doing.
All bets are off for me when it comes to the PA10s and later stuff. My impression - looking in from the outside - is that it all starts to get a bit more laissez faire when the locally-built cars started coming into the mix and Nissan were providing parts and comprehensive data on how to build G2 and G4 cars. There were also far more Works cars going into the mix (by contrast, the Works 240Zs and 260Zs were far, far fewer in number and spread out evenly over a good five years). So I don't bring what I call 'later' activities into the mix when I'm researching the Zs. As far as I'm concerned, post '76, Here Be Dragons.
When the likes of Ford and BMC were swapping identities on cars it was usually done for reasons of convenience. 'Log Book', identity papers, insurance, road tax, MOT test certificate and any scrutineering history etc could be switched from one car to another along with the license plate and chassis plate, with obvious logistical advantages. In the case of the Works 240Zs and (few) 260Zs, this was much harder to do. First of all, outside Japan there simply wasn't an available 'stock' of 240Z & 260Z rally cars to switch identities between, and in the case of two Works cars, each would have its own identity (engraved in the firewall sheetmetal for a start...) with which it crossed borders, so there were no Works-spec cars being built-up outside Japan from fresh 'shells. It's also easy to tell the difference between a proper Works car and a privateer-built example.
But the bigger question is motive. Why would they do it? Obviously when a car was crashed or otherwise put out of action it could theoretically be useful to pin its identity on another car but, in the case of the Works 240Zs and 260Zs, the car you were pinning that identity on would already have had its own identity and carnet. Why not use that? The exception was when cars were sent out of country, and ostensibly back to Japan, with the identities of Works cars attached to them (in order to satisfy the Carnet), but that takes particular Works car identities OUT of the mix rather than keeping them in it. It also caused all sorts of trouble back in Japan. Heads rolled.
These things are for me - generally speaking - only clues in what is in fact the much bigger task of following the careers of genuine Works cars, and what happened to them afterwards.
I've been reading this thread and enjoying it very much, but thought I would 'chip-in' with my two-bob's worth.
Have to say first-up, the info I'm providing here relates to the later PA10 works cars (160J / Stanza etc) and I fully appreciate Alan's info only relates to the "HS30, HLS30, RS30 and RLS30 240Z & 260z rally cars". For many years now I have studied the works PA10s (and learnt an awful lot from Derek who posted earlier) and collected all the info I can, including a clear example of Nissan in Japan re-using a 'Carnet' export plate.
The first pic is the RHD Group 2 SOHC PA10 (Stanza) provided for Ross Dunkerton / Adrian Mortimer in the 1978 Southern Cross Rally. The Japanese plate on this car is TKS57 TE 8013.
Second pic is taken from the Nissan / Datsun Race and Rally Digest No. 4, showing Timo Salonen / Seppo Harjanne celebrating second place outright at the 1980 Acropolis Rally, which they claimed in their LHD Group 2 SOHC Datsun 160J, with Japanese plate TKS57 TE 8013.
The Dunkerton 1978 Southern Cross car stayed in Australia, forming the basis for his 1979 Australian Rally Championship contender until it was rolled in the 1979 Bega Valley Rally. This was likely within its 12-months carnet period, but beyond that event, its fate is unknown. I doubt Nissan in Japan would have repaired the crunched Dunkerton car then converted it to left hand drive so that it would be re-united with its original plate and shipped off to Greece.
As stated clearly above, this is well beyond the Zed period but I did want to show some evidence that plate-swapping by the Japanese works team did take place.
With all due respect, I think the above is a fundamental misapprehension of what the Japanese temporary-export 'Carnet' license plates represented.
The cars were exported for temporary use, so the import duty in the country of use (in this case, Australia) was suspended. The complete 'Carnet' was a complicated and thorough list of the chattels involved in running one or more of these cars outside Japan, usually several pages long and listing parts by piece-by-piece. As long as the car - and theoretically every single piece listed in the full carnet - left the country in question within the time limit of the carnet (usually one year from date of entry) then there was no penalty to pay.
There was no reason or advantage for the Nissan works team to swap one carnet license plate between two cars when they were going in and out of foreign countries. None. Why would you switch a time-limited plate and supporting paperwork (which listed the chassis number, and that chassis number was often 'tagged' with a unique identifier by the customs authority which stamped the carnet, in order to avoid fraudulent use) from one car to another? Once a car had gone back to Japan - usually shagged or at the very least creaking at the seams - there was simply NO reason to use that same number plate again. It simply wouldn't make any logical sense. They could start again with a fresh car and a fresh carnet plate issued by their local licensing authority (Shinagawa, in this case) giving them a clear year of use.
Plate swapping - along with chassis plates and supporting paperwork - were switched by works teams (especially Ford UK and BMC, who did it with gay abandon) in that period, but I have seen no evidence that Nissan's works rally team did it with their HS30, HLS30, RS30 and RLS30 240Z & 260z rally cars. Some of the plates themselves (just the plate) got 'retained' as souvenirs, or when cars DID stay on past their carnet and - all duties paid - they had to be put onto a local registration. These plates sometimes turned up attached to a car, but it was not the Nissan works team who were doing it. They had no reason to.