This is a "tab" which was used to validate prestate plates for 1908. These oval seals were riveted to the prestate plates.

The first modern validation tabs were used in 1943 to conserve metal during WW2.
These were attached to 1942 passenger plates and most 1942 non-passenger types to validate the plates for 1943.

The following year, "44" tabs were issued ... but only for dated-1943 boat plates !

In 1952, a multiyear plate was issued for passenger cars, commercial vehicles, and trailers.
It was dated "52" and had tab slots on either side of the date so a tab could be used.

Annual tabs were used with these plates until 1956 for passenger plates, and until 1960 for non-passenger.

Tabs came in small envelopes with a serial number matching the one on the tabs.

From 1960 to 1971, trailers used blank aluminum tabs with a sticker applied to the tab.
(Apparently the reflective trailer plates did not allow stickers to "stick" very well...)

Some plates with these tabs were used for many years after 1971.