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You will need a mounting plate for the 407k.

Mike, unfortunately you will require an adapter plate. The Holosun sights are not 100% compatible with RMSc-type slide cuts. For reasons that escape me, Holosun did not mill the bottom of the 407K or 507K to accommodate the indexing lugs that are found on the Springfield Hellcat. To mount the Holosun sights, you have two options. The first is to permanently alter your pistol slide by grinding down the lugs. There are companies that offer this service for about $75-100. The second option is to purchase an adaptor plate. An adaptor will allow you to mount the Holosun without machine work on your slide. C&H Precision Slide offers a plate for $45. An adaptor will add a small amount of height and weight to the optic. Sorry but hope this helps.