W & D didn't ring a bell with me, but I bet this is it: Weyerhaeuser and Denkman--who operated a lumber company at 107 4th Avenue in Rock Island. This is the directory entry that was suggested by a reader in an earlier blog post.
It seems reasonable that the Mortiers would have worked for a lumber company in 1888 or so. The 1895 directory indicates they were working for the Rock Island Lumber Company. People may change employers, but the type of work many times remains the same.
More about the history of the Weyerhaeuser Company can be found on its website. Denkmann was Weyerhaeuser's brother-in-law.
This directory is one of thousands recently posted and indexed on Ancestry.com.
The city directories at Ancestry.com can be searched there. I will be posting more about my work in these directories as time allows.