Complex sentence: The first series (the Ob'yekt 765 Sp1, "specification 1"), was produced until 1969 but was replaced by the improved production model, the BMP-1 (the Ob'yekt 765 Sp2), it was produced from 1969 until 1973.
Split into simpler sentences: The first series (Ob'yekt 765 Sp1, "specification 1") was produced until 1969. It was replaced by the improved production model, the BMP-1 (the Ob'yekt 765 Sp2), which was produced from 1969 until 1973.