MontaVista Software is a company selling a number of commercial Linux products and services. Among them very popular Linux distributions specifically targeting embedded devices. At least the original LS1 and LS2 firmware is build on top of MontaVista Linux distribution, likely on top of the Consumer Electronics Edition.
It is unknown who (which company) ported MontaVista Linux to the LS for Buffalo. Judging from the additional software and their quality to be found on the LS (ppc_uartd, mc_ctld, parse_usbscsi, Web Interface, etc.) it was likely Buffalo/Melco, using a MontaVista Linux as a starting point.