Information is pretty scant about the reason the PR91G exists, however this advert does include that it is one of 349, which is a number we've seen before.

This one looks pretty nice; I can't say that the other one is like worth $4,000 more.

The shots were grouped in lots of 4 on the original advert.

This shows lots of great detail of the Fastback, including that familiar PR91 dashboard and driveline, plus the unique features of the rear seats and boot.

These cars have no parcel shelf; the area over the back of the rear seat goes straight into the boot.
From what I can gather both from the photos on the other advert and also from the brochure (which I do not have a good copy of as they seem to hit about $300 when they sell!!!), the rear seat folds down to allow the boot to be enbiggened.

If only they'd thought to hinge the rear window; Isuzu would have designed one of the first hatchbacks on the market!

Find it here on Yahoo Japan:

http://page5.auctions.yahoo.co.jp/jp/auction/e116987712