Nikon 50mm f1.8 Ais lens
 
The 50mm f1.8 is the last version of the manual focus Ais 50's, sometimes called ther 50mm pancake (despite not actually being a pancake lens)
 
Moose Peterson in his difinitive nikon book says that nikon beleive this to have been the finest version 50mm f1.8 they produced.