- 陵戸
From The Cambridge History of Japan, Volume One:
Overview
It's little known that slavery existed in ancient Japanese society-or even in Japan, at all, for that matter. In Nara Japan (710-794 A.D.), Chinese-emulating law compilations such as the Taiho and Yoro Codes set the standard for the ownership of slaves and the subsequent stratification of society. In general, beyond the ranks of aristocrats and ministers, society was separated between the common people (ryoumin) and slaves (senmin). The Cambridge History of Japan Volume One: Ancient Japan informs us that "slaves made up less than 10 percent of the population and were not the country's main producers." Also, one could not bridge the societal gaps through marriage, and crimes of slaves were punished harsher than those committed by normal citizens.
Stratification and Separation within Slavery
The slave class was divided by law into five subgroups according to types of ownership and degrees of freedom. The first subgroup was state slaves (kanko) owned by the central government. They could have families and could use a portion of their labor for themselves. The second subgroup was private slaves (ke'nin) owned by commoners. They had as mch freedom as did state slaves. The third was state chattel slaves (kunuhi) owned by the central government. They were treated as property that could be bought and sold. The fourth subgroup was private chattel slaves (shinuhi) owned by commoners. They were otherwise treated like state chattel slaves. And the fifth was the imperial-mausolea slaves (ryouko) owned by officials. They were used to protect and maintain the tombs of deceased emperors and empresses. Private slaves (ke'nin) are not mentioned in extant household registers or other records of the eighth century, and we find only scattered references to them in legal provisions of that and the following centuries. We therefore assume that private slaves were not numerous and that imperial-mausolea slaves were ranked about as high as commoners were, as they were despised only because of the work they did. Commoners, on the other hand, included artisan groups (shinabe) that were much like slaves. The largest slave subgroup was private chattel slaves, who were owned mainly by temples, shrines, public officials, and wealthy farmers. One private chattel slave, according to contemporary sources, had roughly the value of a strong horse or cow."
http://www.sol.dti.ne.jp/~hiromi/kansei/e_taihoyoro.html says:
「陵戸〔りょうこ〕」
「墓守〔はかもり〕」とも言い、天皇・皇族の陵墓を守る職に従事しています。もとは「陵守（墓守）〔はかもり〕」といって賎身分ではなかったのですが、養老令以降、唐制に倣って「陵戸」として賎身分に編成されます。 良民と同じだけの口分田を班給されていますが、諸陵司の管轄下にあって世襲を強制されており、他の賎や良民との婚姻は禁じられています。
The Imperial Mausoleum Slaves were in a hereditery class system that was (along with the other 4 types of slaves) in contrast with the free, common man (ryoumin). This was in imitation with Tang Chinese law codes. From the above website, there is a chart that explains the relationship that is outlined below in this post.
http://tokyo.cool.ne.jp/human_lights/sources/history/acient-002-j.html says:
七世紀 五色の賤 大宝律令後の日本の身分制度は＜良民｛諸臣・百官人・天下公民（公戸・封戸・品部（ともべ）・雑戸（ざっこ））－賤民（官戸・陵戸・家人・官奴婢・私奴婢）＞という感じで，特に，下層身分である賤民（官戸・陵戸・家人・官奴婢・私奴婢）の５つのを「五色の賤」と言われています。この五色の賤は後に身分解放令（実際は良民が賤民と結婚すると賤民身分となり、税が免除されるなど様々な不都合が生じたためこうした制度をとりやめたと言われる）などで崩壊します。わずかに官奴婢などが「今良」とよばれる身分として形態をかえていきます。
Semmin (slaves), as shown above, were broken down into five categories (五色の賤). 賤民（官戸・陵戸・家人・官奴婢・私奴婢), read: Kanko, Ryouko, Ke'nin, Kunuhi, and Shinuhi, respectively.