There are a couple of ways
1) Data plate bottom line E55 (340) D21 (4sp)
2) Broadcast sheet Line 2 >>> D Trans 21
3) on pass side of trans if original last 8 characters of vin is stamped LM23H [ ORxxxxxx ]

Govier's production code book lists 70 dart swinger 340 all LM23 cars

total production = 13781
shipped (US order) = 10382
3 sp = 955
4 sp = 4423
auto = 5004

Somewhat accurate
hope it helps

What condition is the dart in?
Will be doing a rotiserie resto on mine this fall.