You can compare your Ranger design with the date code on one of the three schematics issued for the Ranger to get a rough idea. You can review the date codes on some of the components to get an idea.
I've never heard or seen any correlation between serial numbers and date of manufacture on their equipment.
The Ranger II was only in production for roughly 4 1/2 years. Check the date codes on some of the components.
Personally, old is old; pick a date.