Springfield to Belchertown.
Cosigned with US 202 in Belchertown.
In 1930, 21 began in Springfield and headed east (Sumner Ave ?) to Parker St, then north along Parker. It met up with its current routing up to Belchertown. It then paired with MA 109 to Enfield, then north to Enfield, North Dana and South Athol. It then went to end in Athol. This routing was maintained until 1939, when the Quabbin Reservoir Site appears on the map and 21 was truncated to Belchertown. The small section from South Athol to Athol appears to have lost its number at that time.
Some time later (I’m not sure exactly when), 21 was truncated to have its southern terminus at US 20.
Restore 21 south of US 20, down Parker St (which feels like a state highway anyways) and into East Longmeadow Center. Extend it down over the length of MA 220 to the CT border. Assuming Connecticut isn’t going renumber their 220 to 21, Massachusetts can renumber their 21 to 220.
Dan Vincent’s MA 21 page