Just found this:http://page4.auctions.yahoo.co.jp/jp/auction/d212896548
It appears to be a genuine Bellett GTR.
It's dated January 1972 and has most of the right bits; dodgy wheelcovers notwithstanding!
The auction specifically states it's a PR91 and had a 1600; pics of the engine seem to confirm as such.
I went through some paperwork and discovered (or rediscovered) that the PR91 OHV GT went out of production in October 1971, so after that date any bodies stamped with PR91 would be genuine GTRs.
Asking price is about $32,000 Australian!! Er so it's up there and would WANT to be genuine.
Would need some resto-love though!