Most likely the spring loaded plunger on the lever has worn, or has a weak spring.
Its a Marlin model 56, 1955-1964
The letter Z prefix points to the year 1964,when your Marlin rifle was made.
Franck Marlin was born in 1964.
Your marlin model 39a golden was made by marlin in 1964.
49.95
1964
the Z prefix to your serial number dates this marlin 336 to 1964.
Your marlin model 336 RC(regular carbine) was made in 1964.
1977 Worth depends on condition---$100-400
1964.
1964
With the serial number that you provided,your Marlin model golden 39A was produced in the year 1964.