Hi, I have an addition to the database.
Using the serial number info from your first post, mine looks to be a 1984 year model Riviera.
Serial Number KD08 077
AH stamped on Rear der. which also points to 1984 (from your link)
Strange codes on Cranks: 4D 2 and 4D 4 are stamped on the back of the crank arms which doesn't match up with any iteration of the Sugino date codes listed on your link[
While it is a 1984 serial number, it is a 1985 model. That style of Nishiki logo was only used on the 1985 models. Typically the manufacturers start building the new models around September. This is a good fit with the September component code and what may be an August serial number. Your bicycle was probably completed around. September-October.
Regarding the crank codes, Sugino was offering double and triple chainring versions of many models during this era. The D may indicate a double version but I'm not positive. The codes may represent February and April 1984.