501

This tube is not listed on either page 3 or 4 of the Philips Type Number Book (radios, rectifiers and ballast tubes). The light bulb catalog no 743E, November 1923, lists the 501 as a "canopy" for the alabasta bulb. (Note; It has been confirmed by Philips that type numbers for both bulbs and tubes were alotted in sequence in the 1920's, as a product was developed, regardless of it being either a bulb, tube, or radio. "Doubling up" with type numbers appears to have occurred at least twice.