I've still got one installed in my 310 as a second com.
It may be legal to use it for receive only but to transmit it needs to meet the 30 ppm frequency requirement. I don't think that the Mark 12 does. When the FAA made more channels available for the allocated aviation spectrum they tightened up on the specs for transmitters to prevent bleeding over from one channel to adjacent channels.
J. Cramer provided the following link in th KX170B thread:
http://wireless.fcc.gov/services/in...stations#National%20Aeronautical%20Corp.The Mark 12 is on the list of unacceptable radios.
