I've just scanned a Japanese Pioneer brochure that may be of interest. Since your Japanese may be a little bit rusty, let me summarize a little of what I've gleaned from the brochure.
In addition to the SPEC line, Pioneer also marketed what they called the "Project" line, at least in Japan. "Project" included a "Project X" line of components very similar in appearance to the SPEC components: the rack-mount C-73, C-1800 (and later, the 1800II) and C-1500 (II) control amplifiers coupled with M-73, M-1800(II) and M-1500(II) power amps. There were also the rack-mount F-73 tuner and F-1500 tuners (apparently there was no F-1800 tuner).
In addition to the "Project X" components there were more traditional component systems:
1) Project 700 and Project 7 systems built around the SA-8800II amplifier,
2) Project 8 (no 800 version) built around the SA-6850 amplifier,
3) Project 700 and Project 7 built around the SA-6800II amplifier,
4) Project 6M built around the SA-6750 amplifier, and
5) Project 300 built around the SA-4400 amplifier.
The main difference between the system numbers seems mostly to do with the styling. The 900,700, and 300 speakers and cabinets are black, while the Project 9,8,7, and 6 are walnut(?) veneer. There are sometimes a few differences in the equipment as well.
Anyone like me who has been confused by some of Pioneer's speaker numbers in the CS series may find this brochure interesting as well. The speaker numbers match the component system:
1) CS-F99 speakers for Project X9,
2) CS-F77 speakers for Project X7,
3) CS-F900 speakers for Project 900,
4) CS-F9 speakers for Project 9,
5) CS-F8 speakers for Project 800
6) CS-F700 speakers for Project 700
7) CS-F7 speakers for Project 7
CS-F6 speakers for Project 6
9) CS-F300 speakers for Project 300
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