We found this circuit with duty cycle adjustment in the May 1988 Radio Electronics magazine. The source must be symmetrical, and the frequency given by R3 and C1.
This is the basic configuration of a monostable TTL with the 74121. The trip is done with an NC...
This fixed source configuration with 78xx series regulators has already been explored in several...
In the figure we have a TTL logic interfacing circuit as CMOS logic using a 6N139 optocoupler...