Manual of Wireless Telegraphy, Manhattan Electrical Supply Company, Number nine (1916), page 110:

Renewal  Audion  Bulbs
Audion Bulb

      Audion Bulbs are sold for renewal purposes only, and then on return of an old bulb or the wing and grid system of an old bulb.
      Audion Bulbs are made in two general style filaments--Tantalum Filament and Hudson Filament.
      Supplied in "S" or standard grade only.
      The Hudson Filament bulbs have from two to three times the operating life of the Tantalum Filament bulbs, and it is therefore more economical to have bulbs with Hudson Filaments.
      All Audion Detectors and Audion Amplifiers listed in this catalogue are regularly furnished with "S" grade Tantalum Filament bulbs.
      Amplifier bulbs are exchangeable only for old Amplifier bulbs, at the prices shown below.
      Amplifier Bulbs are furnished in "X" grade only. We do not recommend shipping Audion Bulbs, either alone or with sets, as they are extremely fragile.
List No.Price.
309  Type "S" Tantalum Filament Audion Bulb$7.00
310  Type "S" Hudson Filament Audion Bulb10.00
297  Tantalum Filament Amplifier Bulbs15.00
298  Hudson Filament Amplifier Bulbs20.00
305  Type "S" Tungsten Special Single Filament Audion Bulb    5.30