I found an excellent guide. Check out INgunowners.com.
Go to DIY gunsmithing forum. There search for 1899 LeFever. You will find and excellent guide as a project write-up from a number of years (2012) back. The author of this extended post has provided an top notch guide with many detailed photos to the disassembly, cleaning, reassembly of the LeFever sideplates shotgun. This should be collated and on a direct link at the LeFever CA.