NEC manufactured their own keyboards and switches as well as source both keyboards and switches from other manufacturers. SMK and Alps have both provided keyboards and switches to NEC.
RKS305E is a designation found on an illuminated NEC reed switch. Based on a failed disassembly attempt, it appears that the return spring was replaced by two sprung conductors that supply power to the lamp, as seen in Omron’s B2x switches. Whether these were ever used in keyboards is presently not known. The meaning of “E” in RKS305E is not known, but Omron used an “E” suffix for their LED-illuminated switches. The lamp type for RKS305E is not depicted in the photographs, and the switches are overpriced at the three places where they are on sale.
Shorter reed switches are known from an unidentified NEC keyboard (PCB code TC-1018). The discovered types are all linear.
There are NEC-branded SMK JM-0400 switches. The one known example is a PC-8801 keyboard of unknown age. As the PCB in that keyboard was made by SMK (from the codes found on the PCB), SMK are likely to have manufactured the switches also.
NEC “SMK-like” is an extremely obscure series that is vaguely similar to SMK JM-0400. The one known example is from September 1986. The series includes alternate action, and Alps click leaves are reported to fit and work correctly.
NEC “oval” is an unidentified series found in various NEC keyboards.
NEC “metal top” is an unidentified and extremely obscure series similar in design to Alps KFL Series.
NEC “SKFR-like” is another obscure switch vaguely similar to Alps SKFR/SKFS Series. There are no known photos depicting their construction. There is not even clear evidence to indicate whether they are discrete or moulded as a single piece, or whether NEC even made them. NEC switches are almost if not always NEC-branded, and there is no apparent branding on this type.
The following text appears in Abstracts of Science and Technology in Japan, 1988:
NEC has developed a new series of keyboard switches. NLL-FP series, using mechanical contacts, has excellent performances such as super-Low profile, 3.5 mm travel and NLL-CN series, using mechanical contacts (same as NLL-FP series), has a tactile feed back mechanism designed ergonomically. The membrane series, using a membrane sheet in the contact section, has dust-sealed and drip-proof structure. This new series is being widely …
This is all that Google Books makes available unless you can guess what words appear in the next snippet. It’s unlikely that the entry was illustrated, and thus far there is no other trace of these switches anywhere, or of any other NEC keyboard switch type. NLL could conceivably be either the metal top or oval type.