Was your car originally titled by a previous owner?
Is it currently titled as a "real" 1957 Lotus 7 or under Illinois' "custom vehicle" catagory?
Illinois has gotten real, shall we say, enthusiastic
about going back to previously registered vehicles and trying to collect additional taxes. It seems that they don't follow their own paperwork process and make it up as they go. They can, and do, pull the title and registration if you don't get this resolved to their satisfaction. I'm glad you got yours straightened out. This story repeated is all over the Web in the Hot Rodding community.