I believe the 387 was an actual coprocessor; the _487_ may be what you're thinking of -- the 487SX contained a whole 486DX inside and just disabled the host processor [1].
I would concur: The 386sx was just 16 bit. The 387sx was for that 16 bit processor. The 486sx was no math co-processor, and the 487 was a whole 486 DX, i.e. the vacancy slot, brought to you by Intel Marketing.