The Denon DL-303 is so rare because it's 30 years old.
There is a review in TAS issue 17, march 1980, pages 42-43.
Beware, this is a light cartridge (5,8 gram) with an unipivot suspension
and a high compliance about 33x10^-6 cm/dyne (at 10 Hz) so you
should use it with a light tone arm.