I put this in a project that doesn’t have Monarch and got the same results as you:
msg.post(".", "set_speed", {kmh = 250})
DEBUG:SCRIPT: hash: [set_speed]
DEBUG:SCRIPT:
{ --[[0000015544431E90]]
speed = 1
}
Reserved messages are discussed in this thread, but “set_speed” is not mentioned. Looks like the sound speed parameter was added in 1.2.162, which is after the thread on reserved words happened and might explain the absence from the list. If indeed my theory is correct.