Designs that never became the Type 64.
Few years after the foundation of the National Police Reserves (later SDF), the upper echelons decided it was time to ditch big American semi-autos and the unreliable (as in literally 0 of the tested rifles passed inspection) pre-mid war Japanese rifles converted to .30 cal with domestically developed weapons. To do this, they got a number of teams within and outside of the NPR/SDF and Howa, calling on former IJA/IJN weapons designers and arsenal heads, especially those who were involved in the development of the Type 4/5 "Japanese Garand" and the Japanese Pedersen. The new rifle was to be light, accurate in full auto, and have a rather low rate of fire of around 400 rounds per minute.
Much of the information is based on websites covering Tsunose Mitsuo's book "The Dream Automatic Rifle ~ Everything Surrounding the Type 64 (幻の自動小銃～64式のすべて)", Arms Magazine articles, but should be taken with some grain of salt as I've not had access to these publications yet and I'm not the best when it comes to translating.
From top to bottom for pic 1:
1958 R1 (Rifle 1 Type) - Gas piston operated with CETME inspired trigger mechanism, designed by Tsunose Mitsuo of the Defence Agency TRDI who had formerly wroked in the IJA Technology HQ (陸軍技術本部?)
1958 R2 (Rifle 2 Type) - Delayed Blowback (system of cams and a cylindrical bolts according to texts) designed by Iwashita Kenzo consul for Howa, who was formerly involved in the development of the Type 99 and Japanese Pedersen copy. Apparently wanted to use the toggle delay mechanism in the R2 but too complex and bulky and was scrapped.
1959 R3 (Rifle 3 Type) - Direct Impingement operated, designed by Howa weapon designer Shizuno Katei. It broke after a few test rounds due to high RoF.
skip R4 (contested - 4 can be read as SHI which is the same way death is read and thus avoided, others say it was an M2 carbine with a fire rate reduction mod) and the R5 (R3 with a gas piston to improve reliability and reduce RoF, blueprint only)
By 1960, politicians and brass in the Defence Agency started to seriously consider adopting the M14 and there were plans for 50,000 rifles to be purchased in the Second Defence build up budget which would start in 1961.
To prevent this, the R6 was developed from the already developed Type 60 12.7mm spotting rifle, and was reduced in calibre and given rate reducer mechanism that the developer claims is seen in the SKS and FN type rifles. At the same time, Tsunose was ordered to leave the Defence Agency and work in Howa, which gave him much better access. The R6 types would go on to be refined, simplified by all the actors involved in the R project and then some (e.g. the K designation in one of the last models of the R6, the R6K, was taken from Iwashita Kenzo who developed a simplified rate reducer that solved reliability issues with previous R6 prototypes). By 1963, the final R6 model (R6E2 Kai) would be sufficient and the budget set aside for the 50,000 M14s were used to purchase the R6EKai, which would be designated the Type 64.
Some sources in Japanese