I tried to look for the serial number on my M-180, but it's not even inside the amp. There's a serial number on the fan, but could it be for the whole amp or just the fan? It's a smudgy paint-stamped thing and it reads "11 1985". Another thing: the top cover is missing. Could the serial number be only on the cover? Are the covers available as spare parts? Paging Mr. Benjamin Strange.