This is the housing insert to allow CZ 455, CZ 457 and CZ 512 rifles to be used with .22LR magazines.
Since the CZ 455 CZ have used a universal rimfire magwell in .22WMR / .17HMR size to allow barrel changes, to size it down for .22LR magazines the insert is required.
Made from CZs proprietary tough black polymer it is held in place with a stainless pin.
CZ-UB part numbers 311233804014 and 273199002666.