Complex sentence: About 600 D.III's and slightly modified D.IIIa's were built between its introduction in August and it's replacement a year later, many of which were still in service at that time, about 450.
Split into simpler sentences: About 600 D.IIIs and slightly modified D.IIIas were built between its introduction in August and its replacement a year later. Many were still in service at that time, about 450.