The 3-641209-8: This is a through-hole, vertical (straight) header with a single row of 8 male pins. It mates with standard MTA-156 receptacle housings and IDC connectors — the 3.96 mm pitch and friction lock are the compatibility checkpoints.
Comparing to Peer Alternatives
The 1318699-3 is another 8-position MTA-156 header but with a different shroud configuration (fully shrouded vs 1-wall) — choose the 1-wall if you need side access for wire dressing or visual inspection of the terminal seating.