Flight International on-line archive has a lot of stuff over the YS-33 / Y-X. It is a long saga that started in 1965 and ended circa 1977 when Japan jumped into the 757 / 767 bandwagon.
https://www.flightglobal.com/pdfarchive/search.aspx?ArchiveSearchForm%24search=YS-33&ArchiveSearchForm%24fromYear=1966&ArchiveSearchForm%24toYear=1977&x=0&y=0
https://www.flightglobal.com/pdfarchive/search.aspx?ArchiveSearchForm%24search=Y-X&ArchiveSearchForm%24fromYear=1966&ArchiveSearchForm%24toYear=1977&x=34&y=16
Looks like they started with a jet-powered YS-11 then moved to a trijet aircraft. By 1977 they ended with a share of 15% into Boeing 767.
https://www.flightglobal.com/pdfarchive/view/1973/1973%20-%201186.html?search=CTDC
The Y-X project was first with the old NAMC company, but it was burdened by debt, so the Japanese government created a whole new entity, the CTDC, in 1973. Oddly enough, NAMC wasn't disbanded until 1982.
Establishment of the Civil
Transport Development Corporation,
with responsibility for the YX project
(as successor to Namco), by the
Japanese Ministry of International
Trade and Industry was approved on
March 30, and the company was
formally created on April 3 (1973)
http://www.jadc.jp/en/about_jadc/outline/