Jim,
The 101.27430 (12x24) and 101.27440 (12x36) were made (or at least advertised) from late Spring or early Summer of 1950 until late in 1957. The first Power Tools catalog to list them was the 1951 which was copyright 06/05/1950. The last was the Fall 1957 Big Book. Unfortunately, with the first significant digit of the serial number missing (they were all 5-digit plus a leading zero), the only way to get any closer is to pull the spindle and read the dates engraved on the bearings. Those are actually bearing receipt inspection dates but they would set the earliest possible date. Actual date could be up to a year later but more likely within 6 months.
Robert D.