Ran another test and apparently the Mk1 Beacons and Salvage Beacons are not quite what I thought they were.
Placed Comm and Caravan Plots at Taffington, after thoroughly looting West Everett, and:
MK1 BEACONS - Dropped a Mk1 Beacon at West Everett and later I got the Mk1 Beacon completion notice, see image. It appears that the Mk 1 Beacon salvages materials that I normally could not pickup, so it is an added bonus. To thoroughly salvage/loot a cleared location you must: (a) loot bodies, containers and stuff laying around, and (b) then drop a Mk 1 Beacon.
UPDATE: MISSED THE OBVIOUS - After days of game time, the NPC at the Starlight caravan plot suddenly unassigned himself so the link was broken. After traveling to Starlight and reassigning an NPC to the caravan plot, the Salvage Beacon is working as expected. SALVAGE BEACON - after receiving the Mk1 Beacon notice, returned to Taffington, and placed loot with a Salvage Beacon in a container at Taffington for transport to Red Rocket. It was not accepted, see image. At the moment, it appears that not only do you (a) need both types of plots at both ends and (b) satisfy the distance limit, but you can only move loot from a location to a settlement, but not from a settlement to another settlement. I will continue testing.
It is also possible that I missed an explanation somewhere - try again