Old-Timer 101 (August 2008) has an article about Mr. Oba of the Bellet GT Club and his collection. Ken is not in any of the pictures, so this article appears to have been written around June of 2008. There is a little about his shop, a page about an Express, barely a mention or photo of his GT Fastback, but some meaty content about his GTX. The GTX cars were the prototypes for the use of the G161W DOHC engine in the Bellett GT body (the GTR), built around 1968 and 1969 for racing, all were painted light blue, and had odd scoops/vents between the B Pillar and the rear wheel well.
Mr. Oba's car was the fifth place finisher in the 1969 Suzuka 12 Hour race held on August 9-10 (232 laps). It was car number 3, driven by Tachi Yonemura and Shogo Omori. The JAF race results list this car as a "Bellett GT", so it is not clear if this car was originally a GTX or not. This becomes relevant because the article explains that the car was previously owned by Kimishige Kohira, the president of Tochigi Isuzu (a dealership/distributor whose website has Bellett and other historic information), and the car was raced in TACS and JCCA events. Tochigi seems to be a semi autonomous subsidiary with a strong pride in Isuzu history who was never forced to disavow Isuzu's car making history.
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This picture is from the Tochigi website, and is pre-restoration.
Mr. Oba acquired the car 8 years ago and converted it over to the specification of Shigeki Asaoka's car in that same Suzuka 12 hour race. That would have been car number 2, driven by Shigeki Asaoka and Kanji Katayama/Katakachiyama, listed in the JAF results as a Bellett GTX, and that car finished in First Place (245 laps).
The GTX is described as having aluminum front and rear fenders (shown flared for wider tires) and FRP (fiberglass) hood and trunk lid. The roof is stated as the only steel panel on the car. The side vents or scoops behind the doors were originally believed to have been for rear brake cooling, but during the restoration, they determined that the vents were solely for differential cooling.
The pictures show the car without bumpers, with orange/red front and rear valence panels, though the rest of the car is light blue with number 2 on the sides, and assorted Isuzu, Dunlop, and NGK sponsor stickers. The car has round front turn signals, long hood, no rear ventilation vents above the rear wheel arches, and the triple horizontal rectangle tail lights. The grille is black mesh with only two regular headlamps and a pair of Bosche rally style (Cibie Oscar style) driving/fog lamps spaced away from the headlamps but not pushed together at the center. The interior is stripped except for the dash (no center console), with a Japanese style roll cage that has been formed around the dash. Those side scoops feed old style accordion hose that attaches to two round fittings where the rear seat once was, blowing down on the differential. The trunk is filled with a large fuel cell that almost touches the underside of the trunk lid. It has to hold 20+ gallons based on the size. Engine is a red valve cover DOHC with two dual side draft carbs (40 mm?). I don't see any statement of the engine power, but one sentence references 7,000 RPM, followed by another referencing 8,000 RPM.
The "discovery" that the rear side vents were actually for differential cooling makes it sound like the vehicle was already in GTX configuration when it was being raced in 1969, and was not converted as a replica. If they had built the vents themselves, there would not have been a discovery as to their true use.