Pending pics, the absence of any brazed on fittings save pump hooks would suggest boom era or older. Serial numbers are not always in the traditonal, BB location. I've seen them stamped on dropouts, head lugs, seat lugs, brake bridges and just about any location of the seat tube.

The intial stage of investigation, besides the serial number and posting pics, would be to check the rear droput spacing, BB thread standard, required seat post diameter and checking for markings on the fork's steerer tube. They can often provide clues to the era, nationality and level, if not the actual manufacturer.