For a house built around 1979, we're usually looking at a torsion spring system that's been replaced once or twice already, and an opener that's had at least one motor swap. We carry standard torsion springs, 7-foot and 8-foot cable sets, and nylon rollers on the truck, because those turn over fast across houses from this era. What we don't carry standing by: proprietary opener boards for a handful of older brand-specific units. If your opener is one of those, we'll order the part and tell you the lead time before we start, not after.
We stock what fits the 1970s-through-80s build pattern because that's what we see most in this part of the service area. If your door is a genuine original from construction, parts can take longer - that's the honest limitation, not a sales pitch.