A floating point unit (FPU) for 386 systems. Back then both the 386SX and 386DX didn't feature a FPU and one could be installed in an socket onto the motherboard specifically for this FPU.

This IIT 4C87DLC looks like it was meant for the 486 but it is not in fact. The FPU came along with this Cyrix Cx486DLC which is some sort of 486 but for 386 systems.

I used the 4C87DLC in conjunction with an Intel 386DX25, AMD Am386DX33 and Cyrix Cx486DLC and in any case it worked fine.