Given the housing stock here, we carry a spread of torsion spring sizes that cover doors from the 1960s and 70s, not just current-build hardware. That includes older wind and wire gauges that a newer-construction supplier might not stock at all.
We also carry standard rollers in both nylon and steel, replacement cables sized for older extension and torsion setups, and hinges in the gauge that shows up on doors from this era, which tends to be heavier than what goes into new builds today.
On the opener side, we bring parts that fit both older chain-drive units still running in some of these houses and the belt-drive openers that have replaced them over the years. We don't carry every panel style - full custom insulated replacements usually get ordered rather than kept on the truck - but for keeping an original door moving another few winters, most of what we need is already on board when we make the nine-mile drive out from Atlanta.