ADE Adapter <- click (link to suggestion made earlier)
I suggest:
Shadow Mount V3.0 for Ruger® Mark™ Series Pistols | TANDEMKROSS
www.tandemkross.com
Edit:
Ok - those are both for the 507c, sorry.
I did find this one:
Low Profile Red-Dot Mount, Ruger Mark Series Holosun 507K
Please Read Compatibility Notice Below! Our new low profile mount for Ruger MARK series pistols allows the end user to mount a Red Dot Optic directly to the frame of the pistol. This allows for a much lower bore axis vs. picatinny rail/mount stack up and has been designed for a clean integrated...lobos-industries.com
Thanks. I have to think this over to see if I want to keep using the 507C I put on it, which means I need something else for the PDP, or order one of those Lobos Industries mounts, spending more money and selling the TK shadow rail that I bought. Or I could put the 507C back on my PDP, put the cheap PA red dot back on the Mark IV Competition and order an Optics Ready Shield Plus, put the 507K on it and sell my old Shield Plus. Decisions decisions...