The production series of locomotives were delivered from 1968, with 55 built in two batches between 1968 and 1975.

The second batch of locomotives, EF66 21 to EF66 55, delivered between 1973 and 1975, incorporated a number of minor changes and improvements.
Most noticeable was the extension of the cab roofs over the windscreens to reduce the deposition of abrasion dust from the pantographs on the windscreens.
Some of the first-batch locomotives (EF66 1 to 20) were also subsequently modified with the "sun visor" style cab end roofs.
The production series of locomotives were delivered from 1968, with 55 built in two batches between 1968 and 1975.
The second batch of locomotives, EF66 21 to EF66 55, delivered between 1973 and 1975, incorporated a number of minor changes and improvements. Most noticeable was the extension of the cab roofs over the windscreens to reduce the deposition of abrasion dust from the pantographs on the windscreens. Some of the first-batch locomotives (EF66 1 to 20) were also subsequently modified with the "sun visor" style cab end roofs.
JNR Class EF66