Hello Kevin,
1. We do not have control over posting of our products on Amazon, eBay...etc.
You are right. MCX413A-BCAT is "ConnectX-4 EN network interface card, 40GbE single-port QSFP"
2. The simple difference: ConnectX4-EN is 100GbE Ethernet Adapter while is ConnectX-4 Lx EN is provides 1/10/25/40/50 gigabit speeds.
If you need to know more technical details please see the product briefs for each cards family.
Product Briefs:
ConnectX-4 EN: http://www.mellanox.com/related-docs/prod_adapter_cards/PB_ConnectX-4_EN_Card.pdf
ConnectX-4 Lx EN: http://www.mellanox.com/related-docs/prod_adapter_cards/PB_ConnectX-4_Lx-EN_Card.pdf
Cheers,
~R