Not all the factories followed the military serial number dating procedure typical of /26\. Your carbine was most likely built in the mid to late 70s.
It would appear that the various factories went through production cycles regarding small arms. During other cycles things such as buses, trucks, bicycles, etc. would be made. Not all factories would produce small arms at once and production was probably shifted around the country for economic and strategic reasons.
Smaller factories also seem to have retained tools from earlier production models, using parts like milled trigger guards and gas tubes, threaded bbls, etc right until the end of T56 production in the 80s, so those features alone cannot be used to gauge the era a carbine was made. The only constant feature of post 1966 production is the use of the spike bayonet.
T56 production was meant to have been phased out in the early 70s in favour of the T63. The T63 was a failure and after some 6 million were made production was stopped in 1978.
T56 production was reintroduced at several factories for a time spanning 1978 to the mid 80s. This also coincided with a period of border wars with Viet Nam.
Shifting doctrine within the PLA at that time was leaning towards the modern mechanised infantry model and away from the previous "people's army" model with it's static defence in depth doctrine and a huge people's militia. The T56 had been retained by the PLA as an ideal battle rifle (at least being deployed as one) for "people's war" and so remained standard until the T81 was well into production and the people's militias had been demobbed in favour of a smaller, mobile army typical of most contemporary modern forces, a process still underway a quarter of a century later. The remains of the militias were reconstituted as the "People's Armed Police".
The last T56s were probably built in the 1978-86 period and many of the "new" military T56s seen here derive from that era.