I hate quoting Wikipeadia becasue anyone can edit it but...
http://en.wikipedia.org/wiki/T-55
T-55AD/T-55AMD/T-55AD1/T-55AMD1 - "D" refers to tanks with Drozd ('thrush') APS. Soviet Naval Infantry saved money by installing Drozd on a small number of tanks instead of opting for appliqué armour, or acquiring newer T-72s. About 250 were kept in stores for secrecy, but later switched to simpler reactive armour.
Don