The complex solution:

1. Tear down the shelves.
2. Rebuild them with greater strength and stiffness.

The medium effort solution:

1. Remove jars.
2. Add vertical in the middle.
3. Put jars back.

The practical solution:

1. Remove jars.
2. Reload the shelves from the bottom up. Put the jars that will be consumed last in first and bottommost.
3. Add shims on top of jars if and where required. The center jars would give themselves to being shimmed.
4. Consume jars from top down.
5. Wash, rinse and repeat if and when new jars are added.

the very simple solution to prevent the sagging shelves now and when it was built?

It's like a Bell curve, only upside down.

(the 'triple layer' effect looks like a layer of fat that has congealed. Whatever was canned was probably cooked with hog jowls or other meat for flavoring.)

What is inside the triple-layer jars on the second shelf from the top? And while we’re at it, I’ve always wondered about those chunky bits in the jars directly underneath. Someone please do the contents ID!

The standard formula for finding deflection y at any given point from one end, say from LHS, is given by:

y = [wx(L-x)/24EIL][L**2+x(L-x)]...(1)

where:
E : Young’s modulus of elasticity of the material from which the beam is fabricated (N/m2)
I : Area Moment of Inertia (m4)
L : Total length of the beam measured between centres of support (m)
x : Some distance from one side of the beam, say from LHS (m)
w : Load (N or kN)

clearly shows that jars should be removed from the ends rather than from the middle.

I wonder if any newspaper reported what happened to the shelves.

Whoa! The Peacocks will no doubt make it through the winter, but I'm not too sure about those shelves. I can hear them groaning across the decades, under the weight of all those goodies. Well done, ladies.

I'm getting nervous just looking at this — of all the time travel fantasies elicited by looking through this site, the desire to reach out a hand and prop up those shelves might be the most pressing.

I figure about 3 pounds per jar and about 300 pounds for the entire wall.

I hope they're talking items from the end of the shelves and not the middle.