Any significant damage, corrosion or repairs will reduce the value of all but the rarest pieces by a half to two thirds. Fakes and Reproductions. There were a lot of fakes and reproductions made in the 1920's and 30's sometimes being stamped with genuine maker's marks. These have now had time to develop significant surface oxide or patination so care is required. Fakes often have an even, chemically created, oxide or corrosion pattern.
Genuine plates have a number of knife marks of uneven depth and distribution on the surface.
Hollowares should not have touch marks or hallmarks or London labels under their bases. Measures should be denominated say gill or pint etc. and have verification marks, usually VR, ER or GR, with a number stamped on the lip.