Yes, disk caching software it is.

For Mac OS 7.1.2 it will improve performance for 68k apps (how many applications for 7.1.2 were PPC native?), particularly on nubus machines.

This seller on ebay has a copy if you are interested:

http://www.ebay.com.au/itm/NEW-Speed-Doubler-2-RAM-Doubler-2-Macintosh-LAST-one-sale-/150865057798?pt=AU_software&hash=item2320421806