Before there were bone containers the various bone graphics were used as drops on monster corpses, in dungeon chests, and several other places in the game. Much like the several graphics of the "fur", they were completely useless. When bone containers were created the graphic was recycled for containers only. The bones of 35 weight which were not containers simply stopped spawning. This happened around 1999? I think.
My time frame could be off just a little but the non-container bones using that graphic went the way of the dodo a VERY LONG time ago. The rarity of the bones reflects the lack of players keeping them and the age of the item. I have seen less then a dozen on Atlantic and maybe 2 or 3 on other shards.