This is the only case I know where the serial number is engraved on the wood of the base, instead of on the head.

 
Remark the evolution of the badge through these four examples.

 

ALFA  ca. 1928

Serial #: 247386

Condition: Working

Sociedad Cooperativa Alfa  is producing in Eibar and Zarauz, (Spain)  since years 20s  to nowadays. This example was Singer 15k licensed

To see Alfa ads in the collection, clik here