The full designator for this device is XD216AA/GL. Another tag on the front of the unit shows 87013.
XD216 is the actual model number. AA is the paint color code, but the meaning of GL is not known at this time.

Judging by the equal size of the  A and B distributor segments, the dual tape reader was a simple time-division multiplex application of the device, rather than a cryptographic application. The device permitted  an increase in transmission speed from 60 words per minute to 120 words per minute. This was achieved by cutting the commutator segments in half then wiring them to two  read heads.

The idea was to cut the five intelligence bit distributor segments into two pieces each.  One reader head is connected to the early segments and the other to the late segments.  Then, if the circuit quality is good enough to support double-speed operation, you can transmit two separate messages per character, per rotation of the distributor.  At the receiving end you simply adjust the range finder of one machine to sample early and the other to sample late, so each sees only the bits intended for it.

Technical manual for this device is: TM11-2222

Front view.
Side view.
Rear view.
Closeup of commutator.
Motor nameplate closeup.
X216 in use with a model 19ASR
All photos in this table by Bob Schaefer

Contributors and Credits:

1) Bob Schaefer,  K6OSM  <Commtekman(at)aol.com>
2) Jim Haynes <jhhaynes@earthlink.net>
3) Don Robert House <k9tty(at)dls.net>)

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Dec 4/12