A door from the mid-70s was built before a lot of the standardized torsion spring sizing that's common now, so the spring on an unmodified Merrydale door often doesn't match a stock replacement off the shelf - we measure wire diameter and wind count rather than guessing by door size. Track hardware from that era also tends to be the older style roller bracket, which doesn't always accept a modern extension-to-torsion conversion without extra fitting. On the climate side, our area runs mild with occasional freezes, which isn't severe weather by most standards, but on 40-to-50-year-old galvanized hardware, repeated freeze-thaw cycling on worn lubrication is enough to stiffen a track or seize a roller that's gone too long without service. None of this means a Merrydale door is in bad shape - plenty of these doors are structurally fine. It means the fix is rarely a universal part, and a tech quoting over the phone without seeing the actual bracket and spring is guessing. We'd rather look at it in person before naming a part number.