Some Portaflex heads won't have a serial # (my '63 B15 doesn't) but you can look at the pot codes to get a rough idea of when your amp was made. The amp should have CTS pots, and they should have a seven digit # starting with 137. The middle two digits are the year, the last two digits are the week the pot was made. You could then estimate that the amp itself was probably built 4-8 weeks after the pots were made, since Ampeg was turning them out pretty quickly back then.
Example: #1376302 (137 = CTS pot, 63 = 1963, 02 = second week of )