Thought of something else to keep a spare of: wheels

[Evolver]

I like to have spare drive belts for everything that I own that needs a belt. I don't want to rely on a pair if Justas nylons for the job.

[Marthony]

This brings to mind logistics/engineering of WWII: Where applicable, it is good to design/acquire hardware that has as many common parts as possible. For this example, the varieties of lube for US vehicles was reduced to less than 10, making logistics of part/supply maintenance more efficient.

It may be a bit much to buy wagons, generators, etc. with compatible wheels, but you get the idea. More practically, one may try to acquire goods that use the same battery size so your preps focus on fewer varieties. For example, dodging items that need C cells in favor of others if you already have hardware that needs D's or AA's.