If you zoom in closely to the center of the plastic base pic (inside the diamond area) there is what looks like a "pentastar" (Mopar) design in the plastic casting.
The "1260B" cast into the base mostly likely may indicate time period of manufacture (???). So WAG - maybe from a 1960-61 Mopar product???
Definitely not original to your trucks that I can find (see screenshot of a reproduction arm rest for 1955-59 for an idea of what the originals looked like).
