[wsh]

RJ45S and RJ45M are ordering codes for so-called “registered jack” configurations for terminal connections to the U.S. telephone network. These codes were defined until 2000 in the FCC Rules (47 CFR § 68.502(e)) and later in the TIA/EIA-IS-968 standard, and they refer to single and multiple arrangements of two wires and a programming resistor on a miniature eight-position keyed jack.

https://www.govinfo.gov/content/pkg/CFR-2000-title47-vol3/pd...

Unfortunately, the “RJ45” part of these codes has become a metonym for the unkeyed version of the miniature eight-position jack and plug, now widely used for Ethernet and other purposes, but strictly speaking, RJ45 refers to a different connector with totally incompatible wiring.