Shin Yasuhara joined Kyocera in 1980 as a designer and worked for them until 1997 when he left to concentrate on designing and manufacturing his own camera. His concept was that of a modern rangefinder able to use classic lenses. He chose to use the Leica Thread Mount giving access to a wide range of lenses. However, his concept was also that a single lens on a modern rangefinder body is all that anyone should need and so he chose to call the camera 一式 (Isshiki), which Google translates as 'complete set' but I take to mean 'all you need'.
Having no manufacturing facilities, he needed to find someone to manufacture the camera and so headed to China to discuss it with, as he put it in his book, "a certain national camera station" (Google translated). He doesn't say in the book who the manufacturer was but it is clear now that it was the Jiangxi Phenix Optical Instrument (Group) Co., Ltd, more commonly known as just Phenix.
Also in his book, Yasuhara-san mentions that, during the 1990's, most Japanese camera manufacturers used overseas labour as it was cheaper. Kyocera, being no different, had most of their cameras manufactured in China. Only the high end models were made in Japan. So it's not surprising that Yasuhara-san headed to China, though he denies any prior connection to Phenix.
Even if Yasuhara-san had no prior connecton to Phenix, he must have known they made Yashica cameras for Kyocera and he knew they had the basic building blocks to create a rangefinder. Quoting from his book, he says "The partner state-owned factory in China manufactured mechanical SLRs. In other words, they already had the basic mechanism for winding the film and activating the shutter. Roughly speaking, remove the single-lens reflex mirror box and the pentaprism and mount the Rangefinder on it. I'm done." (Google translation). And this is pretty much what happened. The chassis of the camera, the winding mechanism and the shutter all come from the Yashica FX-3 Super 2000 camera, which Phenix was making at the time, while the rangefinder came from the Phenix/Seagull 205 camera. Some original design was needed for the lens mount, rangefinder coupling and the metering system and the upper body parts with only minor changes needed to the Yashica chassis.
An agreement was made with Phenix to develop and manufacture the camera. They agreed to finance some of the development costs in return for a share of the profits. Phenix also manufactured their own version of the camera to sell in their domestic market. This version of the camera became known as the JG-50.
The camera was announced and reservations started to be taken in October 1998 with shipments starting in March 1999. Around 3000 pre-sales were taken and it took until the end of 2000 to fulfil them all.
The total number of cameras made is unknown but figures up to 4000 is often quoted. However, my own example has a serial number above 5000. So maybe there was more.
The model code that Yasuhara-san gave to the camera was T981. The 'T' stands for Type and 981 refers to it being his first camera that was made in 1998.
Yasuhara went on to design a second camera, the T012, but it was not particularly successful and Yasuhara Manufacturing ceased trading in 2004. It was resurrected, though, in 2007 and now produces specialised lenses.
Update: Shin Yasuhara died March 22, 2020 at the age of 56. Yasuhara Manufacturing is now permanently closed.