Or more specifically co-processors for the Amstrad Mega PC or Amstrad Mega Plus PC.
387SX co-processors pair up with either the 386SX or the 486SLC. You cannot install a DX type co-processor with an SX chip. I ended up with a US83S87 ULSI Math Coprocessor 68-Pin in a PLCC package rated at 33MHz for my 486SLC-based motherboard.
For the original 386SX-based motherboard I went with an ULSI Math Co SX Advanced Math Coprocessor, it also seems to work well rated at 25MHz. Apparently you can install lower clocked chips with higher clocked CPUs, however you might find that results and errors occur. Ratings are there for a reason, unless you have no option to use anything but a lower clocked chip.
In summary get one if you can. They are getting rarer as the days tick on by. Certainly they may not be of much use or function these days, but I hate seeing an empty slot or socket on a motherboard!