You Do Not need an Adapter plate. This is a direct mount to a 1913 piccatinny rail, simple and easy!!

Hi Richard, this does dot fit a 1913 rail. There is no picatinny mount included, nor is there an adapter plate available for it.
Not sure where the others got their information, but this does not fit a pic rail.

Hi Richard and welcome. This is a direct mount for a 1913 Picatinny Rail with the included Picatinny Mount. No adapter purchase needed. Cheers, stay safe and enjoy!

Good evening to you!! The Meprolight MPO Pro-S Closed Emitter 3 MOA Red Dot uses the RMSc/JPoint Footprint so you will need adapter plates. Hope this helps. Have a good rest of your day.
🇺🇸🇺🇸 'merica' 🇺🇸🇺🇸