I bought a Bellett GT-R in Japan last year and it currently undergoing a full bare metal resto in Australia, with an estimated finish date of November 2020. I was in Japan in Jan/Feb this year and caught up with Norikuni Okazaki ( owner of a blue modified GT) and Akira Ogawa. Akira san worked his whole life at Isuzu, mostly as a mechanic and related fields. He has just retired and is the owner of an orange GT-R. I had him inspect my Bellett at Yokohama to verify its authenticity. He gave me a copy of an Isuzu company internal bulletin document, dated September 1966.
I note in some rather old threads that there has been alot of conjecture about PR91 versus PR91W in terms of VINs and what features define a GT-R.
As some members have correctly stated, the PR91"W" was simply an internal company designation for the GT-R and did not appear in the GT-R chassis numbers stamped on the actual cars. There is also lengthy discussions about the key GT-R features.
The Bulletin is a long document in Japanese but I have translated the salient sections describing the key differences between the GT and the GT-R.
Please find it attached as a word document. The Bulletin date is September 1969 ( not 1966)

Nice! Thanks for posting the document, good to have.
I look forward to seeing your car and your progress with it, if you are happy to share. Welcome to Bellett ownership! At the most sporty level no less.
Cheers, Matt.

Welcome, great seeing you on bellett.net. Your post is a great feed of information.
Dave.