This might be a dumb question, but when the old modules said "For four characters levels 5-10," did that mean they should start at level 5 and will progress to 10 during the adventure?
1e D&D assumed that a party might be anywhere from 3 to 12 characters. Also since different classes progressed at different rates, it was not assumed that characters of comparable ability would be of the same level. Lastly, since it was generally assumed most characters would start from 1st level, it was not assumed that all members of the party would be the same level.
A module with a level range of 5-10 therefore means something quite complicated.
At a high end, it suggests that perhaps 3 10th level characters (presumably a fighter type, a M-U (or Illusionist), and a cleric (or Druid)) with reasonable equipment could face the challenges in the module.
Or that the module might be suitable for a 10th level thief, an 8th level paladin, a 9th level cleric, a 5/7 elven Fighter/M-U, and 5th level drawf fighter that was being played by a player new to the group.
Or that the module might be suitably for a 5th level Paladin, a 5th level ranger, two 6th level clerics, a 7th level thief, two 5th level M-U's, two 5th level fighters, a 5th level drawf fighter/thief, and a 5th level gnome illusionist.
Usually 1e modules summarized this by stating the total expected levels the party should have. So a module for levels 5-10 might suggest that the total party levels should be at least 40, often with a caution that parties composed of mostly lower level characters should be given some small extra resources - a few potions, some hired retainers, etc.
Since I was just looking at it, if you read the inside notes, G2: Glacier Rift of the Frost Giant Jarl assumed a party of 9 characters with about 80 total levels.