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In fact I posted a question to Wolf and he replied that he can produce the coil of any voltage the customer would like to have. That is good. The below is data by Wolf von Langa:

MI-1428 - original version of the RCA 1428 - especially made for Manhatten Opera House (AKA MI-1437) - 100V field-coil motor, 8 section phasing cap and fibrous cone material (may have been hemp paper).
MI-1428A - second version of MI-1428 w/same 8 section phase plug but a new phenolic cone. 13V/1.7A (about 18 Watt) field-coil motor, 8.4 Ohm DCR, voice coil: about 4 Ohm DCR
MI-1428B - the phasing plug was changed to 15 sections. Same phenolic coated silk cone, except modified with cloth suspension and felt damping rings (replacement cone is RCA Stock No. 26414)
MI-1443 - same loudspeaker as the 1428B except the field-coil volatge is about 115 Volt
MI-1429 - 15" woofer made by Lansing Manufacturing Co. (Model 15X29) - 110V field supply (used in RCA Shearer/Photophone systems with Lansing 285 high-frequency driver)
MI-1434 - same as MI-1429 except field-coil motor is 13V
MI-1435 - same as 1428 except painted black and built to MGM specs for use in Shearer systems (13V, about 26 Watt)
MI-1437 - formerly known as MI-1428 prototype 100V field-coil driver - made for Manhatten Opera House
MI-1438 - formerly known as MI-1432 - companion 100V field-coil 15" woofer to the original 1428, also made for Manhatten Opera House
MI-1443 - same as 1428B except 115V field-coil

Parameter A1428WVL 8 Ohm 
Frequency response = 340 Hz – 16 kHz (w/2350 horn)
Power handling = 75 Watt
Sensitivity = 110 dB @1W/1m
Minimum impedance = 8.3 Ohm
Diaphragm = composite paper cone driven by a 2" voice coil
Flux density = up to 23 kGauss (measured in the gap)
Net weight = 15 kG
Diameter = 178 mm
Horn throat diameter = 2"

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