TSEK/KY-68 (DSVT Phone)

© Brooke Clarke,  2012

This is a secure military field telephone.  Digital Subscriber Voice Terminal (DSVT).  It's made to be used with a secure digital switch.
The other end might be another of these secure phones or the KY-78 office secure phone or the KY-99 Advanced Narrowband Digital Voice Terminal (ANDVT).

NSN: 5810-01-082-8404

I was expecting the handset to be either the H-350 or H-250, but it's one I haven't seen before.  Marked:
98230
ON273827
Sonetronics
It has a noise canceling microphone and overall looks like the H-250.  Sonetronics lists the p/n CA-ON273827 as the cable & connector for the H-350 handset, so maybe this is a variation on the H-350.

Front 3/4 View	Back Connectors	Control Panel Top center is the Fill connector
Bottom Instructions	Handset

Cryptography
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GRA-39 Radio Set Remote
KYV-2 Secure Voice Module for PRC-68 family radios
KY-38 Man Portable Secure Voice System
KY-57 Voice Transmission Security Device
KY-65 Voice Encryption Unit 
KY-99 Advanced Narrowband Digital Voice Terminal (ANDVT)
MSC-2001 Voice Encryption Unit (VEU)
Phones
SB-22A  Switchboard
TA-1 Sound Powered Telephone Set
TA-312/PT Telephone Set  (Field Phone)
TA-341/TT Analog touch tone phone
TA-838/TT Telephone Set
TG-5-B Telegraph Set

http://jproc.ca/crypto/ky68.html