I know that IL DMV will often re-label older trailers as home built instead of making the effort of finding an original title. Seem them do it on newer trailers too. Some states do not require a license/title for small trailers so a person can buy one in those states and lose the title over the years because they don't have to license it. Then they sell it to someone from a different state that requires a title. Instead of the 2nd state researching for a title or requiring the new purchaser to get a title which may be near impossible the 2nd state DMV will just do a home built title.
Like your 5er it's obviously not a home built but given its age I can see where a previous owner might have lost the title, if they ever had one.
