The January Jones baby daddy mystery continues ... because the actress left a big, fat BLANK SPOT on her son's birth certificate in the space where she's supposed to name the father.
TMZ obtained the document, filed Friday with the L.A. County Dept. of Public Health, which shows January's son, Xander Dane Jones, was born on Sept. 13, 2011.
Several famous studs have been rumored to be the father -- including "SNL" star Jason Sudeikis, super chef Bobby Flay, "X-Men" director Matthew Vaughn ... and "Seeking Justice" co-star Xander Berkeley.
So far, mom continues to remain mum.