What does the last digit mean in a Piper Arrow model number?
The number notes the model

PA-28R-180 180bhp Cherokee Arrow with the ‘Hershey Bar’ wing.

PA-28R-200 200bhp Arrow II with the lengthened fuselage.

PA-28R-201 200bhp Arrow III with the tapered wing.

PA-28RT-201 200bhp Arrow IV with the tapered wing and T tail.

PA-28RT-201T Turbocharged Arrow IV.

The Arrow III was introduced in 1977 with the tapered wing and larger stabilator.  This improved low speed handling characteristics and increased the total fuel capacity from 50 to 77 gallons.