Context : The main manufacturer of the BMP-1 and its different variants was the Kurgan Machine Building Works (Kurganskiy Mashinostroitelyniy Zavod), but PRP-3 artillery reconnaissance vehicles were produced by the Chelyabinsk Tractor Works (ChTZ) and PRP-4/PRP-4M artillery reconnaissance vehicles were produced by the Rubtsovsk Engineering Works (RMZ). 
Subject : BMP-1 
Relation : service entry
What was the year that BMP-1 entered service?