This is a subcategory of handgun mounts. At Optics Trade, we use the term adapter plate to refer to mounts either:
installed in the place of the rear sight (non-optic-ready handguns) or
those fixed on a milled slide (optic-ready handguns).
In this subcategory, there are also mounting standard converter plates, for example, plates that convert Trijicon RMR mounting standard to the Docter Sight mounting standard.
Before you purchase an adapter plate, make a compatibility check – inspect its top and bottom surface.
Its top surface is usually specified with the name of a red dot sight that has a familiar mounting standard, such as Docter sight, Aimpoint Micro, C-more RTS, Shield RMS/SMS, Trijicon RMR – this is written in the name or in the description of the adapter plate. Some adapter plates are compatible with several mounting standards; they are supplied with removable pins that the user can move from one position to another (adapter plates made by Tony System, Outer Impact, and some other companies).
The mounting surface below is usually specified with the name of the handgun on which it can be mounted. Some handguns, such as Sig Sauer P320, are manufactured in both optic-ready and non-optic-ready versions (depending on the version), so you must be careful to choose the correct mount. Adapter plates designed for non-optic-ready handguns are meant to be installed in the place of the rear sight whereas the adapter plates for optic-ready handguns fit directly into the milled slide of the optic-ready handgun.
Follow the instructions of the handgun manufacturer when mounting or have it mounted by a gunsmith.
Adapter plates are almost exclusively made from aluminium, except for certain components which are (sometimes) made from steel.
They are produced by: