The serial number for a Knabe Baltimore series piano should be found inside the top of the piano where the sound board is. After locating the serial number on the piano, look it up in one of these sites: pianoworld.com and total-piano-care.com.
There are various locations for serial numbers. If you do not find it inside the top of your piano, try to look near the top of the wood frame as it may be stamped on the back of the piano. It may also be located at the top front of the piano’s gold harp or plate, under the opened lid on the ledge, stamped on a little plaque, to the right or to the left. For newer or imported pianos, it may be printed on one of the hammers or on one of the keys, behind the nameboard, inside the piano.