The original 486SX-20 was introduced in early 1991, and according to Alex U. Witkowski was a 486 on the "P648" process with the "Disable Floating Point" bonding option used to disable the FPU. The "487SX" was merely a 486DX with an extra key pin to prevent incorrect installation and another pin that disabled the 486SX. Robert Collins says that "The market is supposed to perceive the '487SX as a coprocessor. A coprocessor can obtain a much higher profit margin than a CPU.", and described it as "a brainchild of marketing people. Even many of the engineers at Intel think it is a stupid idea, and deplore the deceptive marketing technique. The same holds true for many of the field representatives, they think it is a sleazy marketing practice." Red Hill described it as "any 386SX-33 was faster, and even a good 386SX-25 would have run it close.".
But the idea was not bad. From https://groups.google.com/d/msg/comp.sys.ibm.pc.hardware/mNTwkNNdKpo/AVlseoZ0UQ0J, by August 1991 the socket was "supposed to be called "the performance upgrade socket."" From http://www.os2museum.com/wp/486-overdrive/: "In early 1992, Intel ran a teaser ad campaign promising wonders to come in the shape of upgrade processors pluggable into the upgrade socket of 486SX systems." The speed of the 486SX chip was increases to 25 Mhz and later 33 Mhz. With the "P650" shrink and the removal of the FPU from the die completely, the 486SX processor could also be packaged in a surface mount PQFP package (which was cheaper than the PGA package previously used). Intel in mid-1992 sold the new surface mount 486SX at a relatively cheap price to OEMs ($120 per 1000 for 486SX-25 in mid-1992 I think) allowing it to become mainstream. With pre-built systems like Dell, Compaq, and Packard Bell it was very popular. The 487SX socket was renamed the "OverDrive" socket, and "clock doubling" allowed 486DX2 (and later 486DX4) OverDrive processors to be sold by Intel. According to the OS/2 Museum, "486 OverDrives, although never cheap, brought a sizable performance gain in a timely manner with 100% compatibility and minimum hassle."