Very interesting Catshooter! Did you notice whether they had "bushings" installed in the recoil spring tunnel?
I would suspect that the previous owners perhaps experienced reliability issues and sent them back to S&W.
I've only handled/examined one 4516 no dash, the one at the range day. To the best of my recollection it had a single recoil spring. But my oldtimers is firing up on me today.
mbliss57, thanks for the kind words. I'm no expert. Just a shooter.
Best regards, 18DAI