The 1.5SMC510C: The bidirectional channel protects both polarities from a single device — useful on AC lines or DC lines where reverse polarity is a risk.
For a BOM that already uses this device, the existing PCB layout and protection window are a known quantity. If you are designing a new board, look at current Littelfuse TVS families in the DO-214AB package with similar standoff and clamping voltages — the footprint is standard, so a parametric search by voltage and package will surface active alternatives.