Be sure to check on the series number, which is indicated by the first 3 numbers of the serial number (stamped on side of receiver). Some early versions of the Mini-14 were actually a different caliber (.222), and some are not supported by Ruger any longer.
180 Series
Produced from 1974-1977.
Brass ejects upwards.
Shipped from factory with wood handguard installed.
Original Mini-14, no longer supported by Ruger (i.e., no 180-specific parts available)
181 - 186 Series
Produced from 1978-1999.
"Standard" Mini-14
No factory scope mounts, single blade front sight, low round rear peep sight.
Brass ejects upwards.
Still supported by Ruger factory parts.
196 - 197 Series
Produced from 2000-2004.
Same as 181-186 Series.
187 - 188 Series
Produced from 1982-1995.
Mini-14 Ranch Rifle
Factory Scope Mounts, flip-up rear sight, single blade front sight.
Brass ejects to side to avoid contact with scope.
Still supported by Ruger factory parts.
195 - 197 Series
Produced from 1996-2004.
Mini-14 Ranch Rifle.
Same as 187-188 Series.
580 - 582 Series
Produced from 2005-Present.
Mini-14 Ranch Rifle.
Winged Rear Peep Sight, Winged (3-blade) Front Sight.
Integrated Scope Mounts.
At some point in the 580 Series, Ruger introduced the "heavier," flanged/tapered barrel which improved accuracy and decreased heat sensitivity (stringing).