year of david brown 990?

I dont think there is a way of reading the serial number for date of build like you can with Ford tractors.
Below is the build period of some models of 990. It is possible to narrow build dates to some of the changes to the build specs as there is a listed in the back of each section of the repair manuals.

990 Imp 1961-1965 990A, 990B
Serial Nos 440001-480600

990 Selectamatic 990A, 990B
1965-1968 Serial nos 482001-505286

990 Selectamatic 990A, 990B
1968-1971 Serial nos 800001-831351

Production carried on until about 1979 when the 90 series tractors were released.

Ash
December 2009

It will be around 1976

Harley
November 2009