There are several documents in the document center for this, it would help to know what refrigerator
model you have...some models in this era "1973" had a switch in the back that was the changeover from electric operation to gas functions, often a rod would run underneath to the front of the unit to a knob that you would turn inside to toggle the switch one way or another...this mechanism was often a flaw. The switch if not used would seize or freeze up sticking in the electric position, in this position on some models it would block the gas valve from operating by physically blocking the rotation of the gas knob until it would toggle the other way when unfrozen moving it physically unblocking the gas knob allowing its operation...
Also keep in mind the air in the gas line to the refrigerator
takes a long long time to purge out, most often on gas starts where long use of electric functions only were used makes the issues on the gas system restarts just about normal.
A picture of your works would help, snap a photo of the rear access if you cannot determine your model. Feel free to P.M. me for more details...finding a buddy who is savvy or has been there and dun that is helpful too...at least do not trouble shoot alone.
I do not mind step by step suggestions, when testing gas systems a buddy system is good sense along with a near fire extinguisher too...this is not a scare tactic, I am being practical, I do this every time, I also fire people who do not in my employ.
Your system is simple, while its got allot of magic inside psychologically speaking its really a simple unit...In 1973 the design variations were done on the fly by cowboy engineers now 80 years of age...the variations were at times funny.