This is interesting. Mine is 607C9...After some research, I found #224C9 on another forum and in mint condition. The owner said he purchased from the grandson who said it was placed in a console back in 1962. Go figure.
+1 for Cornell. All my McIntosh Gear past and present have all been serviced by him. My most recent service last year was my MA6200. After going thru it , he said its an even 100Watts per channel. The man is a Master!