Labor is typically around 30% of the final cost of prepared food in a restaurant.

Remaining 70% is 30% food costs (which has dropped drastically since the 50s), then 20-30% operations. Profit is whatever is left.

So a diner burger is not mostly labor but I honestly have no idea what these costs were 70 years ago. I'd love to know, seems like something is missing.

Likely everything in the chain going up 1-10%.