For a town built mostly in the mid-1980s, the torsion spring sizes cluster tightly - we carry the wind and wire gauge combinations that fit that generation of door without a special order. Extension springs on older two-spring setups are also common enough here that we keep pairs on the truck rather than ordering them after the fact.
What we don't stock on the truck: oddball springs for custom doors outside the standard residential sizing, and anything built before 1962, since that's outside the service area's housing range entirely. If a door turns out to need one of those, that's a return trip, and we'll say so on-site instead of guessing.
Cable and drum sets get replaced alongside the spring more often than not on these houses, since a snapped torsion spring on a door this age has usually been loading unevenly for a while, and the cable shows it. We'd rather replace both in one visit than have you call back in six months for the cable.