I didn’t read to far into this thread but this is what I know. The 407/507k models follow the shield rmsc footprint. Shield also makes an rms which is a larger optic meant for larger slides. The rmsc is meant for single stacks as well as the 407/507k. If a plate says shield rms and not rmsc it will not allow you to mount a 407/507k
Not sure where you got your info; but you might want to re-read my post #22 just above yours.
The RMS and RMSc have the same bolt mount pattern; the RMS is slightly wider with side battery tray, the RMSc is just a bit narrower that fits micro pistol slides better, but battery tray gets deleted.
Meanwhile....as shown in post #22, the Holosun 'K' series Red Dots should fit the same footprint/bolt pattern but the rear lugs are eliminated and the front lugs are shorter. However, you do get a side battery tray with the HS 'K' series and the controls are vastly superior.
-Vinny