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).

What encryption system does it use, what will it interoperate with? Let me know.

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 left missing ID plate. Top center is the Fill connector Top right is the memory hold up battery cover
Bottom Instructions	Handset

Cryptography
Crypto Machines
EE-8 Telephone Set
Fullerphones
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