The machine Serial Number is stamped into front of the knee casting--not the head (the head numbers are a completely separate series and have nothing to do with machine numbers). To find the correct Serial Number  wind the table inwards as far as possible. The sliding guards should move to expose the number. The number stamped into the head is not the machine number: heads had their own numbers stamped into the main housing and often on a tag attached to the upper section. 
The numbers below apply to the Series models. The special and modified machines - such as Synchro-Trace Duplicators, used a different sequence (and some of these may have been converted back into ordinary types).