Hmm....No sheetmetal, no serial number....the skeptical side of me initially says "was this machine stolen and stripped to hide its identity?"
Obviously if you know the history, this is not the case.... My thing is...With the placement of Serial Number tags these days, theres really no way they just fall off accidently.
Another thing to think about...especially with that machine because they made them for so long...there were so many serial number breaks etc, that you could be in for alot of heartburn at the parts counter if you dont have the SN#