Pretty sure it's red oxide primer ,

these people seem to agree :

[URL unfurl="true"]http://cs.finescale.com/fsm/modeling_subjects/f/3/t/120690.aspx[/URL]

[URL unfurl="true"]https://www.britmodeller.com/forums/index.php?/topic/235044736-accurate-chipping-color-for-soviet-and-american-tanks/[/URL]